set major-version to "9.x" used only for the package installation section (#17562)

closes #17561

Author: jsvd

docs/docset.yml

@@ -42,5 +42,5 @@ subs:
   dlm:   "data lifecycle management"
   dlm-init:   "DLM"
   stack-version: "9.0.0"
-  major-version: "9.0"
+  major-version: "9.x"
   docker-repo: "docker.elastic.co/logstash/logstash"