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
clusterState,
            ClusterInfo.EMPTY,
            SnapshotShardSizeInfo.EMPTY,
            0L
        );
        for (ShardRouting shardRouting : routingAllocation.routingNodes().unassigned()) {
            assertTrue(shardRouting.toString(), shardRouting.unassigned());
            assertThat(
                shardRouting.unassignedInfo().getLastAllocationStatus(),
                equalTo(
                    shardRouting.primary() && shardRouting.shardId().id() == 1
                        ? UnassignedInfo.AllocationStatus.DECIDERS_THROTTLED
                        : UnassignedInfo.AllocationStatus.NO_ATTEMPT
                )
            );
        }
        reconcile(
            routingAllocation,
            new DesiredBalance(
                1,
                randomBoolean()
                    ? Map.of()
                    : Map.of(
                        new ShardId(clusterState.metadata().index(DesiredBalanceComputerTests.TEST_INDEX).getIndex(), 0),
                        new ShardAssignment(Set.of("node-0"), 1, 0, 0)
                    )
            )
        );
        assertTrue(routingAllocation.routingNodesChanged());
        for (ShardRouting shardRouting : routingAllocation.routingNodes().unassigned()) {