Use release drafter v7 (jenkinsci#198)

Explicitly load release drafter base config from GitHub

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

The release drafter documentation (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.

Author: MarkEWaite

.github/release-drafter.yml

@@ -1,5 +1,5 @@
 # see https://github.com/jenkinsci/.github/blob/master/.github/release-drafter.adoc
-_extends: .github
+_extends: github:jenkinsci/.github:/.github/release-drafter.yml
 version-template: $MAJOR.$MINOR.$PATCH
 name-template: $NEXT_PATCH_VERSION
 tag-template: plot-$NEXT_PATCH_VERSION

.github/workflows/release-drafter.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       # Drafts your next Release notes as Pull Requests are merged into "master"
-      - uses: release-drafter/release-drafter@v6
+      - uses: release-drafter/release-drafter@v7
         with:
           publish: ${{ contains(github.event.head_commit.message, '[maven-release-plugin] prepare release') }}
         env: