ci: update dependabot config (add cooldown, single update entry)

Author: nunomaduro

.github/dependabot.yml

@@ -4,16 +4,9 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 5
     groups:
       github-actions:
         patterns:
-          - "*"
-  - package-ecosystem: "github-actions"
-    directory: "/"
-    schedule:
-      interval: "weekly"
-    target-branch: "5.x"
-    groups:
-      github-actions:
-        patterns:
-          - "*"
+          - "*"