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

@@ -10,3 +10,5 @@ updates:
           - "*"  # Group all Actions updates into a single larger pull request
     schedule:
       interval: weekly
+    cooldown:
+      default-days: 7