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
for (int i = 0; i < indexRoutingTable.size(); i++) {
                final var indexShardRoutingTable = indexRoutingTable.shard(i);
                assertTrue(indexShardRoutingTable.primaryShard().initializing());
                assertThat(indexShardRoutingTable.replicaShards().stream().filter(ShardRouting::unassigned).count(), equalTo(3L));
            }
        }
        final var stateWithInitializingReplicas1 = startInitializingShardsAndReroute(allocationService, stateWithInitializingPrimaries);
        for (final var indexRoutingTable : stateWithInitializingReplicas1.routingTable()) {
            for (int i = 0; i < indexRoutingTable.size(); i++) {
                final var indexShardRoutingTable = indexRoutingTable.shard(i);
                assertTrue(indexShardRoutingTable.primaryShard().started());
                assertThat(indexShardRoutingTable.replicaShards().stream().filter(ShardRouting::unassigned).count(), equalTo(2L));
                assertThat(indexShardRoutingTable.replicaShards().stream().filter(ShardRouting::initializing).count(), equalTo(1L));
            }
        }
        final var stateWithInitializingReplicas2 = startInitializingShardsAndReroute(allocationService, stateWithInitializingReplicas1);
        for (final var indexRoutingTable : stateWithInitializingReplicas2.routingTable()) {
            for (int i = 0; i < indexRoutingTable.size(); i++) {
                final var indexShardRoutingTable = indexRoutingTable.shard(i);
                assertTrue(indexShardRoutingTable.primaryShard().started());
                assertThat(indexShardRoutingTable.replicaShards().stream().filter(ShardRouting::unassigned).count(), equalTo(1L));
                assertThat(indexShardRoutingTable.replicaShards().stream().filter(ShardRouting::initializing).count(), equalTo(1L));
                assertThat(indexShardRoutingTable.replicaShards().stream().filter(ShardRouting::started).count(), equalTo(1L));
            }
        }
        final var stateWithInitializingReplicas3 = startInitializingShardsAndReroute(allocationService, stateWithInitializingReplicas2);
        for (final var indexRoutingTable : stateWithInitializingReplicas3.routingTable()) {
            for (int i = 0; i < indexRoutingTable.size(); i++) {
                final var indexShardRoutingTable = indexRoutingTable.shard(i);
                assertTrue(indexShardRoutingTable.primaryShard().started());