Improve dependabot config (#22)

* Add "groups" to dependabot config

Sync the config to https://github.com/django-commons/membership/blame/78870d861b9c9cb416f04213e3332c70b4594c14/.github/dependabot.yml

* Add "cooldown" to dependabot config

Sync the config to https://github.com/django-commons/membership/blame/78870d861b9c9cb416f04213e3332c70b4594c14/.github/dependabot.yml

Author: ulgens

.github/dependabot.yml

@@ -4,5 +4,11 @@ version: 2
 updates:
   - package-ecosystem: github-actions
     directory: /
+    groups:
+      github-actions:
+        patterns:
+          - "*"  # Group all Actions updates into a single pull request
     schedule:
       interval: weekly
+    cooldown:
+      default-days: 7