chore: add cooldown to npm dependabot configuration (#3173)

- Add 15-day cooldown period to npm dependency update configurations

Author: ContentfulCormac

.github/dependabot.yml

@@ -66,4 +66,6 @@ updates:
           - minor
           - patch
         patterns:
-          - '*'
+          - '*'
+    cooldown:
+      default-days: 15