Docs: Flag serve_from_sub_path as available in 6.3 (#17674)

Follow-up to PR #17048. Flag the new serve_from_sub_path option as available in 6.3.

docs/sources/installation/configuration.md

@@ -155,6 +155,7 @@ callback URL to be correct).
 > case add the subpath to the end of this URL setting.
 
 ### serve_from_sub_path
+> Available in 6.3 and above
 
 Serve Grafana from subpath specified in `root_url` setting. By
 default it is set to `false` for compatibility reasons.