I've seen three builds fail now witht he same error and this started happening on Nov 7 so it's likely we've introduced something here:

https://gradle-enterprise.elastic.co/scans/failures?failures.failureClassification=all_failures&failures.failureMessage=Execution%20failed%20for%20task%20%27:x-pack:qa:rolling-upgrade-multi-cluster:v8.6.0%23follower%23oneThirdUpgradedTest%27.%0A%3E%20%60cluster%7B:x-pack:qa:rolling-upgrade-multi-cluster:v8.6.0-follower%7D%60%20failed%20to%20wait%20for%20cluster%20health%20yellow%20after%2040%20SECONDS%0A%20%20%20%20IO%20error%20while%20waiting%20cluster%0A%20%20%20%20%20%20503%20Service%20Unavailable&search.relativeStartTime=P28D&search.timeZoneId=America/Los_Angeles#

```
* What went wrong:
Execution failed for task ':x-pack:qa:rolling-upgrade-multi-cluster:v8.6.0#follower#oneThirdUpgradedTest'.
> `cluster{:x-pack:qa:rolling-upgrade-multi-cluster:v8.6.0-follower}` failed to wait for cluster health yellow after 40 SECONDS
    IO error while waiting cluster
      503 Service Unavailable
```

It seems the follower cluster is having issues coming back up after upgrade.
server/src/test/java/org/elasticsearch/cluster/routing/allocation/allocator/DesiredBalanceReconcilerTests.java
.setNodeId("node-0")
                                .setType(SingleNodeShutdownMetadata.Type.REPLACE)
                                .setTargetNodeName("node-2")
                                .setStartedAtMillis(System.currentTimeMillis())
                                .setReason("test")
                                .build()
                        )
                    )
                )
            ),
            "test",
            ActionListener.noop()
        );
        assertThat(shuttingDownState.getRoutingNodes().node("node-2").shardsWithState(ShardRoutingState.INITIALIZING), hasSize(1));
    }
    public void testRebalance() {
        final var discoveryNodes = discoveryNodes(4);
        final var metadata = Metadata.builder();
        final var routingTable = RoutingTable.builder();
        final var indexMetadata = randomPriorityIndex("index-0", 3, 1);
        metadata.put(indexMetadata, true);
        routingTable.addAsNew(indexMetadata);
        var clusterState = ClusterState.builder(ClusterName.DEFAULT)
            .nodes(discoveryNodes)
            .metadata(metadata)
            .routingTable(routingTable)
            .build();
        final var settings = throttleSettings();
        final var clusterSettings = new ClusterSettings(settings, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);
        final var canAllocateShardRef = new AtomicReference<>(Decision.YES);