Adjust REST test skip version for join field retrieval

The version can be updated now that the test was backported.

modules/parent-join/src/yamlRestTest/resources/rest-api-spec/test/20_parent_join.yml

@@ -101,8 +101,8 @@ teardown:
 ---
 "Test field retrieval":
   - skip:
-      version: " - 7.99.99"
-      reason: bugfix is not yet backported
+      version: " - 7.13.1"
+      reason: bugfix was introduced in 7.13.2
   - do:
       search:
         body: