build: Let dependabot suggest updates to GitHub actions (#8490)

Configure Dependabot to update GitHub Actions in workflows.

.github/dependabot.yml

@@ -1,6 +1,12 @@
 version: 2
 updates:
-- package-ecosystem: gomod
+- package-ecosystem: "github-actions"
+  directory: "/"
+  schedule:
+    interval: weekly
+  open-pull-requests-limit: 10
+  
+- package-ecosystem: "gomod"
   directory: "/"
   schedule:
     interval: weekly