This seems to be related to some unexpected deprecation warning related to system indices, that's why I tagged it as Core/Infra.

**Build scan:**
https://gradle-enterprise.elastic.co/s/cnwwh5dnseay4/tests/:x-pack:qa:full-cluster-restart:v7.9.2%23upgradedClusterTest/org.elasticsearch.xpack.restart.FullClusterRestartIT/testSecurityNativeRealm

**Reproduction line:**
`./gradlew ':x-pack:qa:full-cluster-restart:v7.9.2#upgradedClusterTest' -Dtests.class="org.elasticsearch.xpack.restart.FullClusterRestartIT" -Dtests.method="testSecurityNativeRealm" -Dtests.seed=AEA01868BA18CBDC -Dtests.bwc=true -Dtests.locale=el-GR -Dtests.timezone=EST5EDT -Druntime.java=17`

**Applicable branches:**
master, 8.0

**Reproduces locally?:**
Didn't try

**Failure history:**
https://gradle-enterprise.elastic.co/scans/tests?tests.container=org.elasticsearch.xpack.restart.FullClusterRestartIT&tests.test=testSecurityNativeRealm

**Failure excerpt:**
```
org.elasticsearch.client.WarningFailureException: method [GET], host [http://127.0.0.1:45353], URI [/.security/_settings/index.format], status line [HTTP/1.1 200 OK]
{".security-7":{"settings":{"index":{"format":"6"}}}}

  at __randomizedtesting.SeedInfo.seed([AEA01868BA18CBDC:2558E43B80AE6BC4]:0)
  at org.elasticsearch.client.RestClient.convertResponse(RestClient.java:331)
  at org.elasticsearch.client.RestClient.performRequest(RestClient.java:301)
  at org.elasticsearch.client.RestClient.performRequest(RestClient.java:276)
  at org.elasticsearch.xpack.restart.FullClusterRestartIT.testSecurityNativeRealm(FullClusterRestartIT.java:112)
  at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(NativeMethodAccessorImpl.java:-2)
  at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
  at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:568)
  at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1758)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$8.evaluate(RandomizedRunner.java:946)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$9.evaluate(RandomizedRunner.java:982)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$10.evaluate(RandomizedRunner.java:996)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at org.apache.lucene.util.TestRuleSetupTeardownChained$1.evaluate(TestRuleSetupTeardownChained.java:44)
  at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
  at org.apache.lucene.util.TestRuleThreadAndTestName$1.evaluate(TestRuleThreadAndTestName.java:45)
  at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
  at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:375)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl.forkTimeoutingTask(ThreadLeakControl.java:824)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$3.evaluate(ThreadLeakControl.java:475)
  at com.carrotsearch.randomizedtesting.RandomizedRunner.runSingleTest(RandomizedRunner.java:955)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$5.evaluate(RandomizedRunner.java:840)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$6.evaluate(RandomizedRunner.java:891)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$7.evaluate(RandomizedRunner.java:902)
  at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at org.apache.lucene.util.TestRuleStoreClassName$1.evaluate(TestRuleStoreClassName.java:38)
  at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
  at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at org.apache.lucene.util.TestRuleAssertionsRequired$1.evaluate(TestRuleAssertionsRequired.java:53)
  at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
  at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
  at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
  at org.apache.lucene.util.TestRuleIgnoreTestSuites$1.evaluate(TestRuleIgnoreTestSuites.java:47)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:375)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl.lambda$forkTimeoutingTask$0(ThreadLeakControl.java:831)
  at java.lang.Thread.run(Thread.java:833)

```
x-pack/qa/full-cluster-restart/src/test/java/org/elasticsearch/xpack/restart/FullClusterRestartIT.java
final Request getSettingsRequest = new Request("GET", "/.security/_settings/index.format");
            getSettingsRequest.setOptions(
                expectWarnings(
                    "this request accesses system indices: [.security-7], but in a future major "
                        + "version, direct access to system indices will be prevented by default"
                )
            );
            Response settingsResponse = client().performRequest(getSettingsRequest);
            Map<String, Object> settingsResponseMap = entityAsMap(settingsResponse);
            logger.info("settings response map {}", settingsResponseMap);
            final String concreteSecurityIndex;
            if (settingsResponseMap.isEmpty()) {
                fail("The security index does not have the expected setting [index.format]");
            } else {
                concreteSecurityIndex = settingsResponseMap.keySet().iterator().next();
                Map<?, ?> indexSettingsMap = (Map<?, ?>) settingsResponseMap.get(concreteSecurityIndex);
                Map<?, ?> settingsMap = (Map<?, ?>) indexSettingsMap.get("settings");
                logger.info("settings map {}", settingsMap);
                if (settingsMap.containsKey("index")) {
                    int format = Integer.parseInt(String.valueOf(((Map<?, ?>) settingsMap.get("index")).get("format")));
                    assertEquals("The security index needs to be upgraded", SECURITY_EXPECTED_INDEX_FORMAT_VERSION, format);
                }
            }
            
            createUser(false);
            createRole(false);
        }
        assertUserInfo(isRunningAgainstOldCluster());
        assertRoleInfo(isRunningAgainstOldCluster());
    }