ci: adding GitLab mirroring GitHub Action

Mirror creates new references such as branches and tag. It updates existing
references and it deletes references when they no longer exist in the source but
do in the mirror.

* https://git-scm.com/docs/git-push#Documentation/git-push.txt---mirror
* https://christoph.ruegg.name/blog/git-howto-mirror-a-github-repository-without-pull-refs

.github/workflows/mirroring.yml

@@ -0,0 +1,17 @@
+name: Mirroring
+
+on: [push, delete]
+
+concurrency:
+  group: ${{ github.workflow }}
+
+jobs:
+  GitLab:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code.
+        run: git clone --mirror "https://github.com/${GITHUB_REPOSITORY}.git" "${GITHUB_WORKSPACE}"
+      - name: Get GitLab repository.
+        run: echo "REPOSITORY_NAME=$(echo "${GITHUB_REPOSITORY}" | sed "s|^${GITHUB_REPOSITORY_OWNER}/||g")" >> "${GITHUB_ENV}"
+      - name: Mirroring.
+        run: git push --mirror "https://oauth2:${{ secrets.GITLAB_PERSONAL_ACCESS_TOKEN }}@gitlab.com/DeveloperC/${REPOSITORY_NAME}"