Deprecate loading of solr.xml from ZK

In SOLR-7735, five years ago (committed by me) we allowed loading of solr.xml from zookeeper.

Part of the motivation back then was to be able to put solr.xml in a central place and not being forced to initialize SOLR_HOME with that file, similar to the goal of SOLR-9575. And perhaps also an easy way to update solr.xml.

However, I think it may have been a mistake that should be deprecated and ultimately removed. Why? The solr.xml file is the "node config" for solr. Where you set node-level props that need to be there at early node startup. So even if you change the file in ZK, nodes will have not way of reloading those plugins dynamically without a restart

The file may even contain local-node-specific settings that shuold differ from node to node. Especially during rolling upgrade, it may be important to be able to have a different solr.xml on the new nodes and on the old ones. With property substitution you can do some of that, but you cannot add XML tags with a sysprop.