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
public void allocateUnassigned(
                ShardRouting shardRouting,
                RoutingAllocation allocation,
                UnassignedAllocationHandler unassignedAllocationHandler
            ) {
                allocateUnassigned.accept(allocation, unassignedAllocationHandler);
            }
            @Override
            public void afterPrimariesBeforeReplicas(RoutingAllocation allocation) {}
        };
    }
    private static ShardsAllocator createShardsAllocator() {
        return new ShardsAllocator() {
            @Override
            public void allocate(RoutingAllocation allocation) {
                var dataNodeId = allocation.nodes().getDataNodes().values().iterator().next().getId();
                var unassignedIterator = allocation.routingNodes().unassigned().iterator();
                while (unassignedIterator.hasNext()) {
                    unassignedIterator.next();
                    unassignedIterator.initialize(dataNodeId, null, 0L, allocation.changes());
                }
            }
            @Override
            public ShardAllocationDecision decideShardAllocation(ShardRouting shard, RoutingAllocation allocation) {
                throw new AssertionError("only used for allocation explain");
            }
        };
    }
}