Use doc instead of _doc in FullClusterRestartIT

ES does not accept doc type starting with underscore until 6.2.0. We
have to use "doc" instead of "_doc" in FullClusterRestartIT if we are
upgrading from a 6.2.0- cluster.

Closes elastic#42581

qa/full-cluster-restart/src/test/java/org/elasticsearch/upgrades/FullClusterRestartIT.java

@@ -1031,7 +1031,6 @@ public void testSoftDeletes() throws Exception {
      * This test creates an index in the old cluster and then closes it. When the cluster is fully restarted in a newer version,
      * it verifies that the index exists and is replicated if the old version supports replication.
      */
-    @AwaitsFix(bugUrl="https://github.com/elastic/elasticsearch/issues/42581")
     public void testClosedIndices() throws Exception {
         if (isRunningAgainstOldCluster()) {
             createIndex(index, Settings.builder()
@@ -1043,7 +1042,7 @@ public void testClosedIndices() throws Exception {
             if (randomBoolean()) {
                 numDocs = between(1, 100);
                 for (int i = 0; i < numDocs; i++) {
-                    final Request request = new Request("POST", "/" + index + "/_doc/" + i);
+                    final Request request = new Request("POST", "/" + index + "/" + type + "/" + i);
                     request.setJsonEntity(Strings.toString(JsonXContent.contentBuilder().startObject().field("field", "v1").endObject()));
                     assertOK(client().performRequest(request));
                     if (rarely()) {