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
} while (changed);
        for (final var shardRouting : clusterState.routingTable().allShards()) {
            assertTrue(shardRouting.started());
            assertThat(shardRouting.currentNodeId(), oneOf("node-0", "node-1"));
        }
        assertSame(clusterState, allocationService.reroute(clusterState, "test", ActionListener.noop())); 
                                                                                                          
        desiredBalance.set(desiredBalance(clusterState, (shardId, nodeId) -> nodeId.equals("node-2") || nodeId.equals("node-3")));
        canRebalanceGlobalRef.set(Decision.NO);
        assertSame(clusterState, allocationService.reroute(clusterState, "test", ActionListener.noop())); 
                                                                                                          
        canRebalanceGlobalRef.set(Decision.YES);
        canRebalanceShardRef.set(Decision.NO);
        assertSame(clusterState, allocationService.reroute(clusterState, "test", ActionListener.noop())); 
                                                                                                          
        canRebalanceShardRef.set(Decision.YES);
        canAllocateShardRef.set(Decision.NO);
        assertSame(clusterState, allocationService.reroute(clusterState, "test", ActionListener.noop())); 
                                                                                                          
        canAllocateShardRef.set(Decision.YES);
        
        
        final var reroutedState = allocationService.reroute(clusterState, "test", ActionListener.noop());
        assertThat(reroutedState.getRoutingNodes().node("node-0").shardsWithState(ShardRoutingState.RELOCATING), hasSize(1));
        assertThat(reroutedState.getRoutingNodes().node("node-1").shardsWithState(ShardRoutingState.RELOCATING), hasSize(1));
    }
    private static void reconcile(RoutingAllocation routingAllocation, DesiredBalance desiredBalance) {
        new DesiredBalanceReconciler(desiredBalance, routingAllocation, new NodeAllocationOrdering()).run();
    }
    private static AllocationService createTestAllocationService(