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
.nodes(DiscoveryNodes.builder().add(node1).add(node2).localNodeId(node1.getId()).masterNodeId(node2.getId()))
            .build();
        ClusterServiceUtils.setState(clusterService, noLongerMaster);
        newMasterElected.countDown();
        try {
            assertTrue(listenersCalled.await(10, TimeUnit.SECONDS));
        } finally {
            clusterService.close();
            terminate(threadPool);
        }
    }
    private static DiscoveryNode createDiscoveryNode(String nodeId) {
        var transportAddress = buildNewFakeTransportAddress();
        return new DiscoveryNode(
            nodeId,
            nodeId,
            UUIDs.randomBase64UUID(random()),
            transportAddress.address().getHostString(),
            transportAddress.getAddress(),
            transportAddress,
            Map.of(),
            Set.of(DiscoveryNodeRole.MASTER_ROLE, DiscoveryNodeRole.DATA_ROLE),
            Version.CURRENT
        );
    }
    private static IndexMetadata createIndex(String name) {
        return IndexMetadata.builder(name)
            .settings(
                Settings.builder()
                    .put(SETTING_NUMBER_OF_SHARDS, 1)