[Test] Fix rolling upgrade test to only wait for yellow in a mixed cluster

If the primary shard of an index with (number_of_replicas > 0) ends up on a new node in a mixed cluster, the replica cannot be allocated to the old node as
the new node might have written segments that use a new postings format or codec that is not available on the older node.
As x-pack automatically creates indices with number_of_replicas > 0, for example monitoring-data-*, the test can only wait for yellow in a mixed cluster.

Original commit: elastic/x-pack-elasticsearch@945d9e3811

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/mixed_cluster/10_basic.yaml

@@ -2,7 +2,10 @@
 "Index data and search on the mixed cluster":
  - do:
      cluster.health:
-        wait_for_status: green
+        # if the primary shard of an index with (number_of_replicas > 0) ends up on the new node, the replica cannot be
+        # allocated to the old node (see NodeVersionAllocationDecider). x-pack automatically creates indices with
+        # replicas, for example monitoring-data-*.
+        wait_for_status: yellow
         wait_for_nodes: 2
         timeout: 25s
 

qa/rolling-upgrade/src/test/resources/rest-api-spec/test/old_cluster/10_basic.yaml

@@ -6,7 +6,7 @@
         body:
           settings:
             index:
-              number_of_replicas: 0
+              number_of_replicas: 1
 
  - do:
      bulk: