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
.put(SETTING_NUMBER_OF_REPLICAS, 0)
                    .put(SETTING_INDEX_VERSION_CREATED.getKey(), Version.CURRENT)
            )
            .build();
    }
    private static AllocationService createAllocationService(
        DesiredBalanceShardsAllocator desiredBalanceShardsAllocator,
        GatewayAllocator gatewayAllocator
    ) {
        return new AllocationService(
            new AllocationDeciders(List.of()),
            gatewayAllocator,
            desiredBalanceShardsAllocator,
            () -> ClusterInfo.EMPTY,
            () -> SnapshotShardSizeInfo.EMPTY
        );
    }
    private static GatewayAllocator createGatewayAllocator() {
        return createGatewayAllocator(DesiredBalanceShardsAllocatorTests::initialize);
    }
    private static void initialize(RoutingAllocation allocation, ExistingShardsAllocator.UnassignedAllocationHandler handler) {
        handler.initialize(allocation.nodes().getLocalNodeId(), null, 0L, allocation.changes());
    }
    private static GatewayAllocator createGatewayAllocator(
        BiConsumer<RoutingAllocation, ExistingShardsAllocator.UnassignedAllocationHandler> allocateUnassigned
    ) {
        return new GatewayAllocator() {
            @Override
            public void beforeAllocation(RoutingAllocation allocation) {}
            @Override