Update Dependabot configuration

* Remove Gradle as the dependencies are managed manually
* Group the GitHub Actions dependency updates to avoid
  PR spam

.github/dependabot.yml

@@ -1,10 +1,11 @@
 version: 2
 updates:
-  - package-ecosystem: "gradle"
-    directory: "/"
-    schedule:
-      interval: "monthly"
   - package-ecosystem: "github-actions"
-    directory: "/"
+    directory: "/.github"
+    groups:
+      dependencies:
+        applies-to: version-updates
+        patterns:
+          - "*"
     schedule:
-      interval: "monthly"      
+      interval: "monthly"