validate dependabot, update actions

.github/dependabot.yml

@@ -3,8 +3,16 @@ updates:
   - package-ecosystem: "cargo"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "monthly"
     groups:
-      all-dependencies:
+      all-dependencies-cargo:
+        patterns:
+          - "*"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      all-dependencies-actions:
         patterns:
           - "*"

.github/workflows/dependabot-validate.yml

@@ -0,0 +1,19 @@
+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
+        id: validate
+      - uses: marocchino/sticky-pull-request-comment@v2
+        if: always()
+        with:
+          header: validate-dependabot
+          message: ${{ steps.validate.outputs.markdown }}