chore: add cooldown to npm dependabot configuration

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

Author: ContentfulCormac

.github/workflows/dependabot.yaml .github/workflows/dependabot.yml.github/workflows/dependabot.yaml renamed to .github/workflows/dependabot.yml

@@ -15,3 +15,6 @@ updates:
     commit-message:
       prefix: build
       include: scope
+
+    cooldown:
+      default-days: 15