feat(ci): add dependabot to keep GH Actions up to date

Limit bot to update only monthly to avoid spam and save resources.
Also group to one PR to limit PR spam.

.github/dependabot.yaml

@@ -0,0 +1,12 @@
+# https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    groups:
+      github-actions:
+        patterns:
+          - "*"