CI: make dependabot monitor .github/actions directory

and also set `cooldown`, `groups`

related: #19747
closes:  #19756

Signed-off-by: Ozaki Kiichi <gclient.gaap@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

.github/dependabot.yml

@@ -1,8 +1,16 @@
 version: 2
 updates:
   - package-ecosystem: "github-actions"
-    directory: "/"
+    directories:
+      - "/"
+      - "/.github/actions/**/*"
     schedule:
       interval: "weekly"
     commit-message:
       prefix: "CI"
+    cooldown:
+      default-days: 7
+    groups:
+      github-actions:
+        patterns:
+          - "*"