Use GitHub Action for Dependabot (#314)

.github/mergify.yml

@@ -1,11 +0,0 @@
-pull_request_rules:
-  - name: dependabot
-    conditions:
-      - author=dependabot[bot]
-    actions:
-      queue:
-        name: default
-        method: squash
-queue_rules:
-  - name: default
-    conditions: []

.github/workflows/dependabot.yaml

@@ -0,0 +1,13 @@
+name: dependabot
+on: pull_request
+permissions:
+  contents: write
+  pull-requests: write
+jobs:
+  merge:
+    runs-on: ubuntu-latest
+    if: github.actor == 'dependabot[bot]'
+    steps:
+      - run: gh pr merge --auto --squash ${{ github.event.pull_request.html_url }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}