### CI Link

https://gradle-enterprise.elastic.co/s/yp56iltyyinha

### Repro line

./gradlew ':x-pack:plugin:ilm:qa:multi-node:javaRestTest' --tests "org.elasticsearch.xpack.ilm.ChangePolicyforIndexIT.testChangePolicyForIndex" -Dtests.seed=8825024EAE3089DD -Dtests.locale=en-PH -Dtests.timezone=Asia/Karachi -Druntime.java=16

### Does it reproduce?

No

### Applicable branches

7.17 (probably all 8+)

### Failure history

_No response_

### Failure excerpt

This is rather concerning
```
java.lang.AssertionError: policy step registry cache failed sanity check
	at org.elasticsearch.xpack.ilm.PolicyStepsRegistry.getStep(PolicyStepsRegistry.java:378) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleRunner.getCurrentStep(IndexLifecycleRunner.java:108) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleRunner.runPolicyAfterStateChange(IndexLifecycleRunner.java:430) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleService.triggerPolicies(IndexLifecycleService.java:416) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleService.clusterChanged(IndexLifecycleService.java:319) ~[?:?]
	at org.elasticsearch.cluster.service.ClusterApplierService.callClusterStateListener(ClusterApplierService.java:573) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.callClusterStateListeners(ClusterApplierService.java:559) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.applyChanges(ClusterApplierService.java:519) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.runTask(ClusterApplierService.java:428) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.access$000(ClusterApplierService.java:56) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService$UpdateTask.run(ClusterApplierService.java:154) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:718) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:262) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:225) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630) ~[?:?]
	at java.lang.Thread.run(Thread.java:831) [?:?]
[2022-03-15T09:34:09.657115416Z] [BUILD] Stopping node
```
x-pack/plugin/ilm/src/test/java/org/elasticsearch/xpack/ilm/PolicyStepsRegistryTests.java
.put("index.number_of_replicas", 0)
                    .put("index.version.created", Version.CURRENT)
                    .put(LifecycleSettings.LIFECYCLE_NAME, "policy")
                    .build()
            )
            .build();
        SortedMap<String, LifecyclePolicyMetadata> metas = new TreeMap<>();
        metas.put("policy", policyMetadata);
        PolicyStepsRegistry registry = new PolicyStepsRegistry(metas, null, null, REGISTRY, client, null);
        Step step = registry.getStep(indexMetadata, InitializePolicyContextStep.KEY);
        assertNotNull(step);
    }
    public void testGetStepUnknownStepKey() {
        Client client = mock(Client.class);
        Mockito.when(client.settings()).thenReturn(Settings.EMPTY);
        LifecyclePolicy policy = LifecyclePolicyTests.randomTimeseriesLifecyclePolicyWithAllPhases("policy");
        LifecyclePolicyMetadata policyMetadata = new LifecyclePolicyMetadata(policy, Collections.emptyMap(), 1, randomNonNegativeLong());
        String phaseName = randomFrom(policy.getPhases().keySet());
        Phase phase = policy.getPhases().get(phaseName);
        PhaseExecutionInfo pei = new PhaseExecutionInfo(policy.getName(), phase, 1, randomNonNegativeLong());
        String phaseJson = Strings.toString(pei);
        LifecycleAction action = randomValueOtherThan(MigrateAction.DISABLED, () -> randomFrom(phase.getActions().values()));
        Step step = randomFrom(action.toSteps(client, phaseName, MOCK_STEP_KEY, null));
        LifecycleExecutionState.Builder lifecycleState = LifecycleExecutionState.builder();
        lifecycleState.setPhaseDefinition(phaseJson);
        IndexMetadata indexMetadata = IndexMetadata.builder("test")
            .settings(
                Settings.builder()
                    .put("index.number_of_shards", 1)
                    .put("index.number_of_replicas", 0)