Add soft deletes setting in retention lease test

This commit adds a missing soft deletes setting that was lost when the
commit was rebased on the latest 6.x branch before pushing. I am sorry.

Closes #38764

Author: jasontedor

server/src/test/java/org/elasticsearch/index/seqno/RetentionLeaseIT.java

@@ -127,13 +127,13 @@ public void testRetentionLeasesSyncedOnAdd() throws Exception {
         }
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/38764")
     public void testRetentionLeaseSyncedOnRemove() throws Exception {
         final int numberOfReplicas = 2 - scaledRandomIntBetween(0, 2);
         internalCluster().ensureAtLeastNumDataNodes(1 + numberOfReplicas);
         final Settings settings = Settings.builder()
                 .put("index.number_of_shards", 1)
                 .put("index.number_of_replicas", numberOfReplicas)
+                .put(IndexSettings.INDEX_SOFT_DELETES_SETTING.getKey(), true)
                 .build();
         createIndex("index", settings);
         ensureGreen("index");