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
assertTrue(replicaGetter.apply(state6, lowerIndex, 1).unassigned());
        final var state7 = startInitializingShardsAndReroute(allocationService, state6);
        assertTrue(replicaGetter.apply(state7, higherIndex, 0).started());
        assertTrue(replicaGetter.apply(state7, higherIndex, 1).initializing());
        assertTrue(replicaGetter.apply(state7, lowerIndex, 0).unassigned());
        assertTrue(replicaGetter.apply(state7, lowerIndex, 1).unassigned());
        final var state8 = startInitializingShardsAndReroute(allocationService, state7);
        assertTrue(replicaGetter.apply(state8, higherIndex, 0).started());
        assertTrue(replicaGetter.apply(state8, higherIndex, 1).started());
        assertTrue(replicaGetter.apply(state8, lowerIndex, 0).initializing());
        assertTrue(replicaGetter.apply(state8, lowerIndex, 1).unassigned());
        final var state9 = startInitializingShardsAndReroute(allocationService, state8);
        assertTrue(replicaGetter.apply(state9, higherIndex, 0).started());
        assertTrue(replicaGetter.apply(state9, higherIndex, 1).started());
        assertTrue(replicaGetter.apply(state9, lowerIndex, 0).started());
        assertTrue(replicaGetter.apply(state9, lowerIndex, 1).initializing());
        final var state10 = startInitializingShardsAndReroute(allocationService, state9);
        assertTrue(replicaGetter.apply(state10, higherIndex, 0).started());
        assertTrue(replicaGetter.apply(state10, higherIndex, 1).started());
        assertTrue(replicaGetter.apply(state10, lowerIndex, 0).started());
        assertTrue(replicaGetter.apply(state10, lowerIndex, 1).started());
    }
    public void testUnassignedRespectsDesiredBalance() {
        final var discoveryNodes = discoveryNodes(5);
        final var metadata = Metadata.builder();
        final var routingTable = RoutingTable.builder();
        for (var i = 0; i < 5; i++) {
            final var indexMetadata = randomPriorityIndex("index-" + i, between(1, 5), between(0, 4));
            metadata.put(indexMetadata, true);
            routingTable.addAsNew(indexMetadata);