DS-3695, we upgrade the DSpace 7 codebase to Solr v7.3.1
Unfortunately, since that time, Solr v7.3.1 is also considered EOL: https://lucene.apache.org/solr/downloads.html#about-versions-and-support
At a minimum, we should upgrade to v7.7.x (which is still supported). However, this seems like a good opportunity to simply upgrade to v8 (currently v8.5.1) as that's the current version. Additionally, since we'll already be needing to do a major Solr update in DSpace v7, it seems reasonable to just jump directly to Solr v8.