SolrJ 9x vs 8x

As I was looking to merge master into ishan/validations branch, I realized that the master branch no longer works (neither with Solr 8x nor 9x). I see a plethora of errors. All of those started ever since we upgraded SolrJ from 8.11 to 9.1 in PR #57.

I see that all active development is now happening at solr-8.x branch. An 8x version of SolrJ can handle talking to Solr 8x and 9x both, so I think that's the way to go forward to support both.

Should we abandon master branch and make solr-8.x branch the new master? @patsonluk @justinrsweeney @hiteshk25 .
That will allow us to upgrade the open source graphs (mostly.cool/graph.html) to use the latest solr-bench commits (esp. the metrics graphs in a modal window), also allow the merging of the validations branch.