Use new _extends syntax for release drafter

Explicitly load release drafter base config from GitHub (#118)

Release drafter v7 no longer accepts .github as a valid extension.

https://github.com/release-drafter/release-drafter/blob/master/docs/configuration-loading.md#fetching-from-a-repo-named-github says

  The github: prefix is used internally to recognize you want to
  explicitly fetch from a remote (using octokit) instead of loading a
  file on the runtime's filesystem.

.github/release-drafter.yml

@@ -1,5 +1,5 @@
 # Configuration for Release Drafter: https://github.com/toolmantim/release-drafter
-_extends: .github
+_extends: github:jenkinsci/.github:/.github/release-drafter.yml
 # We use the 2-digit versioning in Jenkins weekly releases.
 version-template: $MAJOR.$MINOR
 name-template: $NEXT_MINOR_VERSION