Update moodle-ci.yml: require upgrade-500 branch

Author: bluetom

.github/workflows/moodle-ci.yml

@@ -11,4 +11,4 @@ jobs:
     uses: Opencast-Moodle/moodle-workflows-opencast/.github/workflows/moodle-ci.yml@main
     with:
       requires-tool-plugin: true
-      branch-tool-plugin: main
+      branch-tool-plugin: upgrade-500