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
allocationServiceRef.set(allocationService);
        clusterService.submitUnbatchedStateUpdateTask("test", new ClusterStateUpdateTask() {
            @Override
            public ClusterState execute(ClusterState currentState) {
                var indexMetadata = createIndex("index-1");
                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 -> { throw new AssertionError("Should not happen in test"); },
                        exception -> listenersCalled.countDown()
                    )
                );
            }
            @Override
            public void clusterStateProcessed(ClusterState initialState, ClusterState newState) {
                clusterStateUpdatesExecuted.countDown();
            }
            @Override
            public void onFailure(Exception e) {
                throw new AssertionError(e);
            }
        });
        
        assertTrue(clusterStateUpdatesExecuted.await(5, TimeUnit.SECONDS));
        var noLongerMaster = ClusterState.builder(clusterService.state())