Use dependabot for dependency upgrades

Issue gh-11

Author: sjohnr

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+
+updates:
+  - package-ecosystem: github-actions
+    target-branch: main
+    directory: /
+    schedule:
+      interval: weekly
+    labels:
+      - 'type: dependency-upgrade'

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

@@ -0,0 +1,27 @@
+name: Auto-Merge Dependabot PR
+
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    runs-on: ubuntu-latest
+    if: ${{ github.event.pull_request.user.login == 'dependabot[bot]' && github.repository == 'spring-io/spring-gradle-build-action' }}
+    steps:
+      - id: dependabot-metadata
+        name: Dependabot Metadata
+        uses: dependabot/[email protected]
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+      - name: Enable auto-merge for Dependabot PRs
+        if: ${{ steps.dependabot-metadata.outputs.update-type == 'version-update:semver-patch' }}
+        env:
+          PR_URL: ${{github.event.pull_request.html_url}}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: gh pr merge --auto --rebase "$PR_URL"
+      - uses: actions/checkout@v4
+      - name: Update v2 branch
+        run: git checkout v2 && git merge main && git push