chore: enable Dependabot for GitHub Actions (#883)

Add a github-actions package-ecosystem entry to Dependabot config so
GitHub Actions workflow dependencies are kept up to date (weekly).

Author: inponomarev

.github/dependabot.yml

@@ -14,3 +14,8 @@ updates:
     - dependency-name: com.puppycrawl.tools:checkstyle
       versions:
         - ">= 10"
+- package-ecosystem: github-actions
+  directory: "/"
+  schedule:
+    interval: weekly
+  open-pull-requests-limit: 10