CI: Set up Dependabot on this repo (to update external GitHub Actions)

Author: DilumAluthge

.github/dependabot.yml

@@ -0,0 +1,15 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    groups:
+      # Group all GitHub Actions PRs into a single PR:
+      all-github-actions:
+        patterns:
+          - "*"
+    directory: "/"
+    schedule:
+      interval: "monthly"
+    open-pull-requests-limit: 100
+    labels:
+      - "dependencies"
+      - "github-actions"