only one PR for GitHub actions updates (#150)

Author: ranocha

.github/dependabot.yml

@@ -5,3 +5,8 @@ updates:
     directory: "/" # Location of package manifests
     schedule:
       interval: "monthly"
+    groups:
+      # Group all GitHub Actions PRs into a single PR:
+      all-github-actions:
+        patterns:
+          - "*"