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/main/java/org/elasticsearch/cluster/routing/allocation/allocator/DesiredBalanceReconciler.java
}
                                    continue nextShard;
                                }
                                case THROTTLE -> isThrottled.set(true);
                                case NO -> {
                                    if (logger.isTraceEnabled()) {
                                        logger.trace("Couldn't assign shard [{}] to [{}]", shard.shardId(), desiredNodeId);
                                    }
                                }
                            }
                        }
                    }
                }
                if (logger.isTraceEnabled()) {
                    logger.trace("No eligible node found to assign shard [{}] amongst [{}]", shard, assignment);
                }
                final UnassignedInfo.AllocationStatus allocationStatus;
                if (assignment == null || assignment.isIgnored(shard.primary())) {
                    allocationStatus = UnassignedInfo.AllocationStatus.NO_ATTEMPT;
                } else if (isThrottled.get()) {
                    allocationStatus = UnassignedInfo.AllocationStatus.DECIDERS_THROTTLED;
                } else {
                    allocationStatus = UnassignedInfo.AllocationStatus.DECIDERS_NO;
                }
                unassigned.ignoreShard(shard, allocationStatus, allocation.changes());
                if (shard.primary() == false) {
                    
                    while (i < primaryLength - 1 && comparator.compare(primary[i], primary[i + 1]) == 0) {
                        unassigned.ignoreShard(primary[++i], allocationStatus, allocation.changes());
                    }