ci(dependabot): fix cooldown property name (#1702)

The `cooldown` block expects `default-days`, not `default`. Using the
wrong key fails schema validation, causing Dependabot to silently fall
back to the previously valid config — which still targets
`release/struts-6-8-x` instead of `support/struts-6-x-x`.

Also adds a 3-day cooldown to the `main` maven entry for consistency.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

.github/dependabot.yml

@@ -8,13 +8,15 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 3
     target-branch: "main"
   - package-ecosystem: "maven"
     directory: "/"
     schedule:
       interval: "weekly"
     cooldown:
-      default: 4
+      default-days: 4
     target-branch: "support/struts-6-x-x"
     ignore:
       - dependency-name: "org.eclipse.jetty:jetty-maven-plugin"