Mark test library/sbt plugin updates as internal

Author: xerial

.github/release-drafter.yml

@@ -56,6 +56,7 @@ autolabeler:
   - label: 'library-update'
     body:
       - '/library-update/'
-  - label: 'test-library-update'
+  - label: 'internal'
     body:
       - '/test-library-update/'
+      - '/sbt-plugin-update/'