Fix type name in testSystemIndexMetadataIsUpgraded (elastic#63506)

This test used _doc as the mapping type name, which needs to be set
to doc for versions prior to 6.7.0. This commit fixes the test to use
the proper type name for the current BWC version.

Author: gwbrown

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

@@ -1461,7 +1461,6 @@ public void testRecoveryWithTranslogRetentionDisabled() throws Exception {
     }
 
     @SuppressWarnings("unchecked")
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/63457")
     public void testSystemIndexMetadataIsUpgraded() throws Exception {
         final String systemIndexWarning = "this request accesses system indices: [.tasks], but in a future major version, direct " +
             "access to system indices will be prevented by default";
@@ -1473,7 +1472,7 @@ public void testSystemIndexMetadataIsUpgraded() throws Exception {
 
             Request bulk = new Request("POST", "/_bulk");
             bulk.addParameter("refresh", "true");
-            bulk.setJsonEntity("{\"index\": {\"_index\": \"test_index_old\", \"_type\" : \"_doc\"}}\n" +
+            bulk.setJsonEntity("{\"index\": {\"_index\": \"test_index_old\", \"_type\" : \"" + type + "\"}}\n" +
                 "{\"f1\": \"v1\", \"f2\": \"v2\"}\n");
         if (isRunningAgainstAncientCluster() == false) {
             bulk.setOptions(expectWarnings(RestBulkAction.TYPES_DEPRECATION_MESSAGE));