[ML] ML BWC tests cannot run against pre-ML versions (#37136)

Author: davidkyle

x-pack/qa/full-cluster-restart/src/test/java/org/elasticsearch/xpack/restart/MlMigrationFullClusterRestartIT.java

@@ -73,6 +73,12 @@ private void createTestIndex() throws IOException {
     }
 
     public void testMigration() throws Exception {
+        // v5.5.0 is the first version where ml jobs can be
+        // retained after upgrade
+        if (getOldClusterVersion().before(Version.V_5_5_0)) {
+            return;
+        }
+
         if (isRunningAgainstOldCluster()) {
             createTestIndex();
             oldClusterTests();