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
final 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 desiredBalance = desiredBalance(clusterState, (shardId, nodeId) -> true);
        final var assignReplicas = new AtomicBoolean(false);
        final var allocationService = createTestAllocationService(
            routingAllocation -> reconcile(routingAllocation, desiredBalance),
            new SameShardAllocationDecider(settings, clusterSettings),
            new ReplicaAfterPrimaryActiveAllocationDecider(),
            new ThrottlingAllocationDecider(settings, clusterSettings),
            new AllocationDecider() {
                @Override
                public Decision canAllocate(ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation) {
                    return (shardRouting.primary() && node.nodeId().equals("node-0")) || assignReplicas.get() ? Decision.YES : Decision.NO;
                }
            }
        );
        final TriFunction<ClusterState, IndexMetadata, Integer, ShardRouting> primaryGetter = (state, indexMetadata, shardId) -> state
            .routingTable()
            .shardRoutingTable(indexMetadata.getIndex().getName(), shardId)
            .primaryShard();
        final var state1 = startInitializingShardsAndReroute(allocationService, clusterState);
        assertTrue(primaryGetter.apply(state1, higherIndex, 0).initializing());
        assertTrue(primaryGetter.apply(state1, higherIndex, 1).unassigned());
        assertTrue(primaryGetter.apply(state1, lowerIndex, 0).unassigned());
        assertTrue(primaryGetter.apply(state1, lowerIndex, 1).unassigned());