Add cooldown to Dependabot configuration

Delay automatic dependency updates to allow time for discovery of
regressions or supply-chain compromises in newly released versions.

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

Author: maennchen

.github/dependabot.yml

@@ -7,3 +7,5 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 7