Auto approve & merge dependabot patch upgrade PRs (#30191)

Adapted from the [Github documentation][doc]. This version
approves all patch upgrades to any dependency. Not sure if
we have any dependency that must always be reviewed manually.

[doc]: https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

.github/workflows/dependabot-pr.yml

@@ -0,0 +1,25 @@
+name: Approve & merge successful dependabot patch upgrade PRs
+on: pull_request
+
+permissions:
+  content: write
+  pull-requests: write
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    if: ${{ github.actor == 'dependabot[bot]' }}
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@v1
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+      - name: Approve the PR & enable auto-merge
+        if: ${{ steps.metadata.outputs.update-type == 'version-update:semver-patch' }}
+        run: |
+          gh pr review --approve "$PR_URL"
+          gh pr merge --auto --merge "$PR_URL"
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}