ci: use action to validate dependabot.yml (#1642)

following [this
comment](#1628 (comment))
from @kristovatlas

This action seems to only validate the JSON schema. For full validation,
it's on GitHub to provide this -- tracked in this issue:
dependabot/dependabot-core#4605

.github/workflows/dependabot-validate.yml

@@ -0,0 +1,13 @@
+name: dependabot validate
+
+on:
+  pull_request:
+    paths:
+      - '.github/dependabot.yml'
+      - '.github/workflows/dependabot-validate.yml'
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: marocchino/validate-dependabot@v2