[github_actions] Add automatic updating of github-actions (flutter#5919)

* Revert "Revert "Add automatic updating of github-actions (flutter#5880)" (flutter#5917)"

This reverts commit 8ded25d.

* Update dependabot.yml

Shortened prefix from "github_actions" to "gh_actions" in order to meet the 15 character max length requirement

Author: drewroengoogle

.github/dependabot.yml

@@ -287,3 +287,11 @@ updates:
     schedule:
       interval: "daily"
     open-pull-requests-limit: 10
+    
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    commit-message:
+      prefix: "[gh_actions]"
+    schedule:
+      interval: "daily"
+    open-pull-requests-limit: 10