De-couple SolrJ required Java version from server Java version (main)

Solr 9.x requires Java 11, both for server and solrj client.

In Solr 10 we will likely bump required java version to Java 17, or maybe even 21, and since we are a standalone app we can do that - on the server-side.

However, to give SolrJ client a broadest possible compatibility with customer application environments, we should consider de-coupling SolrJ's java requirement from the server-side. That would allow us to be progressive on the server side Java without forcing users to stay on latest Java in their apps.

I don't know if it makes much sense to be compatible too far back on EOL java versions, but perhaps let SolrJ stay one LTS version behind the server for broad compatibility.