CI: Add Dependabot cooldown for github-actions

The github-security-report security scan flags the github-actions
ecosystem for a missing update cooldown, which is mandatory (any value
passes) and also required by the zizmor workflow auditor. Add a 7-day
default cooldown, matching the global-jjb configuration, so bump PRs
wait before opening on a freshly shipped action release.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Change-Id: I0af57813eca00351154c061a7f9679e600159903
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>

Author: askb

.github/dependabot.yml

@@ -13,6 +13,8 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 7
     groups:
       actions-deps:
         patterns: