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(shardRouting.toString(), shardRouting.unassigned());
            assertThat(
                shardRouting.unassignedInfo().getLastAllocationStatus(),
                equalTo(
                    
                    shardRouting.primary()
                        ? shardRouting.shardId().id() == 1
                            ? UnassignedInfo.AllocationStatus.DECIDERS_THROTTLED
                            : UnassignedInfo.AllocationStatus.DECIDERS_NO
                        : UnassignedInfo.AllocationStatus.NO_ATTEMPT
                )
            );
        }
    }
    public void testUnassignedPrimariesBeforeUnassignedReplicas() {
        
        final var discoveryNodes = discoveryNodes(2);
        final var metadata = Metadata.builder();
        final var routingTable = RoutingTable.builder();
        final var indexMetadata0 = randomPriorityIndex("index-0", 1, 1);
        metadata.put(indexMetadata0, true);
        routingTable.addAsNew(indexMetadata0);
        final var indexMetadata1 = randomPriorityIndex("index-1", 1, 1);
        metadata.put(indexMetadata1, true);
        routingTable.addAsNew(indexMetadata1);
        final var clusterState = ClusterState.builder(ClusterName.DEFAULT)
            .nodes(discoveryNodes)
            .metadata(metadata)
            .routingTable(routingTable)
            .build();