The Solr schema.xml file should be generated programmatically using update-fields.sh

[pdurbin]

update-fields.sh has been around for a while, added in back in #8130 but we (or at least, I) haven't fully embraced it when it comes to editing conf/solr/schema.xml.

schema.xml should be created the same way every time, using Docker so that we don't have differences between Mac and Linux.

In the future, diffs of schema.xml will be easier to grok if we create the file the same way every time.

For context, I'm reviewing #10887 which will result in a change to schema.xml.

I'll make a pull request with a schema.xml file created in Docker and some docs for developers.