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/DesiredBalanceShardsAllocatorTests.java
},
            reconcileAction
        );
        var allocationService = createAllocationService(desiredBalanceShardsAllocator, gatewayAllocator);
        allocationServiceRef.set(allocationService);
        class CreateIndexTask extends ClusterStateUpdateTask {
            private final String indexName;
            private CreateIndexTask(String indexName) {
                this.indexName = indexName;
            }
            @Override
            public ClusterState execute(ClusterState currentState) throws Exception {
                var indexMetadata = createIndex(indexName);
                var newState = ClusterState.builder(currentState)
                    .metadata(Metadata.builder(currentState.metadata()).put(indexMetadata, true))
                    .routingTable(RoutingTable.builder(currentState.routingTable()).addAsNew(indexMetadata))
                    .build();
                return allocationService.reroute(newState, "test", ActionListener.wrap(response -> {
                    assertThat(
                        "All shards should be initializing by the time listener is called",
                        clusterService.state().getRoutingTable().index(indexName).primaryShardsUnassigned(),
                        equalTo(0)
                    );
                    assertThat(reconciliations.get(), equalTo(1));
                    listenersCalled.countDown();
                }, exception -> { throw new AssertionError("Should not happen in test", exception); }));
            }
            @Override
            public void clusterStateProcessed(ClusterState initialState, ClusterState newState) {
                clusterStateUpdatesExecuted.countDown();