Fix GHA on legacy branches

.github/workflows/moodle-plugin-ci.yml

@@ -42,7 +42,7 @@ jobs:
           # Ideally, we would checkout ${{ matrix.moodle-branch }} here.
           # But when doing a plugin upgrade, this would let the build fail.
           # Thus we checkout always the master branch and hope that the steps to start up the Bitnami container won't change too often.
-          ref: master
+          ref: main
           path: bitnami-container
 
       - name: Start Bitnami LDAP