SolrInfoMBean-s category and hierarchy cleanup

SOLR-9947 changed categories of some of SolrInfoMBean-s, and it also added an alternative view in JMX, similar to the one produced by SolrJmxReporter.

Some changes were left out from that issue because they would break the back-compatibility in 6.x, but they should be done before 7.0:

• remove the old JMX view of SolrInfoMBean-s and improve the new one so that it's more readable and useful.
• in many cases SolrInfoMBean.getName() just returns a FQCN, but it could be more informative - eg. for highlighter or query plugins this could be the symbolic name of a plugin that users know and use in configuration.
• top-level categories need more thought. On one hand it's best to minimize their number, on the other hand they need to meaningfully represent the functionality of components that use them. SOLR-9947 made some cosmetic changes, but more discussion is necessary (eg. QUERY vs. SEARCHHANDLER)
• we should consider removing some of the methods in SolrInfoMBean that usually don't return any useful information, eg. getDocs, getSource() and getVersion().