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
public void testNoChangesOnEmptyDesiredBalance() {
        final var clusterState = DesiredBalanceComputerTests.createInitialClusterState(3);
        final var routingAllocation = new RoutingAllocation(
            new AllocationDeciders(List.of()),
            clusterState.mutableRoutingNodes(),
            clusterState,
            ClusterInfo.EMPTY,
            SnapshotShardSizeInfo.EMPTY,
            0L
        );
        reconcile(routingAllocation, new DesiredBalance(1, Map.of()));
        assertFalse(routingAllocation.routingNodesChanged());
    }
    public void testFailsNewPrimariesIfNoDataNodes() {
        final var clusterState = ClusterState.builder(DesiredBalanceComputerTests.createInitialClusterState(3))
            .nodes(
                DiscoveryNodes.builder()
                    .add(
                        new DiscoveryNode(
                            "master",
                            buildNewFakeTransportAddress(),
                            Map.of(),
                            Set.of(DiscoveryNodeRole.MASTER_ROLE),
                            Version.CURRENT
                        )
                    )
                    .localNodeId("master")
                    .masterNodeId("master")
                    .build()