Mute failing CCR retention lease unfollow test

This commit mutes a CCR retention lease unfollow test that is failing
randomly, but frequently.

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/CcrRetentionLeaseIT.java

@@ -460,6 +460,7 @@ public class CcrRetentionLeaseIT extends CcrIntegTestCase {
         }
     }
 
+    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/39185")
     public void testUnfollowFailsToRemoveRetentionLeases() throws Exception {
         final String leaderIndex = "leader";
         final String followerIndex = "follower";