chore(github): Add workflow to mirror git to drupal.org

Author: klausi

.github/workflows/drupalorg-mirror.yml

@@ -0,0 +1,14 @@
+name: Mirror to drupal.org
+on:
+  push:
+    branches: [ 8.x-4.x ]
+jobs:
+  git-mirror:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: wearerequired/git-mirror-action@v1
+        env:
+          SSH_PRIVATE_KEY: ${{ secrets.SSH_PRIVATE_KEY }}
+        with:
+          source-repo: 'git@github.com:drupal-graphql/graphql.git'
+          destination-repo: 'git@git.drupal.org:project/graphql.git'