Allow third_party/java_src/gtld/ to use :mockito2_for_third_party

In [] a change would have been made to your project that is incompatible with
your open source integration. To make sure the open source variant of your
project remains working, we have eagerly updated your open source copy to use
Mockito 2. This CL integrates that change into [].

Please read []and
understand the consequences of this change.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=238445356

javatests/google/registry/module/backend/BUILD

@@ -17,11 +17,11 @@ java_library(
         "//java/google/registry/module/backend",
         "//java/google/registry/request",
         "//javatests/google/registry/testing",
-        "//third_party/java/mockito",
         "@com_google_truth",
         "@com_google_truth_extensions_truth_java8_extension",
         "@javax_servlet_api",
         "@junit",
+        "@org_mockito_core",
     ],
 )