Update plugin_docs_test.yaml

Author: jsvd

.github/workflows/plugin_docs_test.yaml

@@ -22,6 +22,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: elastic/logstash
+          ref: 8.x # test on 8.x until we find how to test with the new docs system
           path: logstash
       - name: Checkout logstash-docs repo
         uses: actions/checkout@v4