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
Consumer<RoutingAllocation> allocationConsumer,
        AllocationDecider... allocationDeciders
    ) {
        return createTestAllocationService(
            allocationConsumer,
            () -> ClusterInfo.EMPTY,
            () -> SnapshotShardSizeInfo.EMPTY,
            allocationDeciders
        );
    }
    private static AllocationService createTestAllocationService(
        Consumer<RoutingAllocation> allocationConsumer,
        ClusterInfoService clusterInfoService,
        SnapshotsInfoService snapshotsInfoService,
        AllocationDecider... allocationDeciders
    ) {
        final var allocationService = new AllocationService(new AllocationDeciders(List.of(allocationDeciders)), new ShardsAllocator() {
            @Override
            public void allocate(RoutingAllocation allocation) {
                allocationConsumer.accept(allocation);
            }
            @Override
            public ShardAllocationDecision decideShardAllocation(ShardRouting shard, RoutingAllocation allocation) {
                throw new AssertionError("should not be called");
            }
        }, clusterInfoService, snapshotsInfoService);
        allocationService.setExistingShardsAllocators(Map.of(GatewayAllocator.ALLOCATOR_NAME, new NoOpExistingShardsAllocator()));
        return allocationService;
    }
    private static class NoOpExistingShardsAllocator implements ExistingShardsAllocator {