Add auto-merge for Dependabot PRs

This will make it easier to maintain the repository. Since this is a
demo project, it is enough to pass CI to update the dependency.

Do not mindlessly copy this approach to projects on which other
projects depend.

.github/workflows/dependabot-auto-merge.yml

@@ -0,0 +1,16 @@
+---
+name: Dependabot auto-merge
+on: pull_request
+
+permissions:
+  contents: write
+
+jobs:
+  auto-merge:
+    runs-on: ubuntu-latest
+    if: github.actor == 'dependabot[bot]'
+    steps:
+      - name: Enable auto-merge for Dependabot PRs
+        run: gh pr merge --auto --merge "${{ github.event.pull_request.html_url }}"
+        env:
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}