.github/dependabot.yml: set cooldown period

Set Dependabot cooldown period to 7 days.

Link: https://docs.zizmor.sh/audits/#dependabot-cooldown

Author: ldv-alt

.github/dependabot.yml

@@ -4,4 +4,6 @@ updates:
     directory: /
     schedule:
       interval: monthly
+    cooldown:
+      default-days: 7
     open-pull-requests-limit: 2