Always validate dependabot config in CI

Current upstream validator is still under work and tracked as
dependabot/dependabot-core#4605.

.github/workflows/validate-dependabot.yaml

@@ -0,0 +1,19 @@
+name: dependabot validate
+
+on:
+  pull_request:
+    paths:
+      - '.github/dependabot.yml'
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: marocchino/validate-dependabot@v3
+        id: validate
+      - uses: marocchino/sticky-pull-request-comment@v2
+        if: always()
+        with:
+          header: validate-dependabot
+          message: ${{ steps.validate.outputs.markdown }}