ci: add dependabot automerge

Author: xobotyi

.github/workflows/ci.yaml

@@ -40,3 +40,15 @@ jobs:
           token: ${{ secrets.CODECOV_TOKEN }}
           files: coverage/lcov.info
           fail_ci_if_error: true
+
+  dependabot-merge:
+    name: "Dependabot automerge"
+    runs-on: ubuntu-latest
+    needs: [ "test" ]
+    permissions:
+      pull-requests: write
+      contents: write
+    steps:
+      - uses: fastify/github-action-merge-dependabot@v3.0.0
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}