Mute MlMappingsUpgradeIT testMappingsUpgrade (#61909)

For #61908

Author: davidkyle

x-pack/qa/rolling-upgrade/src/test/java/org/elasticsearch/upgrades/MlMappingsUpgradeIT.java

@@ -42,6 +42,7 @@ protected Collection<String> templatesToWaitFor() {
      * The purpose of this test is to ensure that when a job is open through a rolling upgrade we upgrade the results
      * index mappings when it is assigned to an upgraded node even if no other ML endpoint is called after the upgrade
      */
+    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/61908")
     public void testMappingsUpgrade() throws Exception {
 
         switch (CLUSTER_TYPE) {