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
assertTrue(shard0RoutingTable.primaryShard().initializing());
            assertThat(shard0RoutingTable.primaryShard().currentNodeId(), equalTo("node-0"));
            final var shard1RoutingTable = stateWithOneInitializingPrimary.routingTable().shardRoutingTable("index-0", 1);
            assertTrue(shard1RoutingTable.primaryShard().unassigned());
        }
        
        allocationFilter.set((shardId, nodeId) -> true);
        final var stateStillWithOneInitializingPrimary = allocationService.reroute(stateWithOneInitializingPrimary, "test", unused);
        {
            final var shard0RoutingTable = stateStillWithOneInitializingPrimary.routingTable().shardRoutingTable("index-0", 0);
            assertTrue(shard0RoutingTable.primaryShard().initializing());
            assertThat(shard0RoutingTable.primaryShard().currentNodeId(), equalTo("node-0"));
            final var shard1RoutingTable = stateStillWithOneInitializingPrimary.routingTable().shardRoutingTable("index-0", 1);
            assertTrue(shard1RoutingTable.primaryShard().unassigned());
        }
        
        allocationFilter.set((shardId, nodeId) -> nodeId.equals("node-1"));
        final var stateWithBothInitializingPrimaries = allocationService.reroute(stateStillWithOneInitializingPrimary, "test", unused);
        {
            final var shard0RoutingTable = stateWithBothInitializingPrimaries.routingTable().shardRoutingTable("index-0", 0);
            assertTrue(shard0RoutingTable.primaryShard().initializing());
            assertThat(shard0RoutingTable.primaryShard().currentNodeId(), equalTo("node-0"));
            final var shard1RoutingTable = stateWithBothInitializingPrimaries.routingTable().shardRoutingTable("index-0", 1);
            assertTrue(shard1RoutingTable.primaryShard().initializing());
            assertThat(shard1RoutingTable.primaryShard().currentNodeId(), equalTo("node-1"));
        }
    }
    public void testMoveShards() {
        final var discoveryNodes = discoveryNodes(4);
        final var metadata = Metadata.builder();