Set Dependabot cooldown period to 4 days

This addresses the zizmor findings by setting a cooldown period of 4 days
for all package ecosystems in dependabot.yml.

Related to: ENG-3236

Author: oschwald

.github/dependabot.yml

@@ -1,7 +1,9 @@
 version: 2
 updates:
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: daily
-      time: "14:00"
+  - package-ecosystem: github-actions
+  directory: /
+  schedule:
+    interval: daily
+    time: '14:00'
+  cooldown:
+    default-days: 4