Keep also the actions up-to-date with dependabot (#466)

Keep also the actions up-to-date following this approach:
https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot#example-dependabotyml-file-for-github-actions

Author: mofterdinger

.github/dependabot.yml

@@ -27,3 +27,7 @@ updates:
       all-npm-dependencies:
         patterns: 
         - "*"
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: "weekly"