More structured management of CHANGES.txt

### Description

There's been interesting patch in Solr that aims at making CHANGES.txt management better [1]. This file gets out of sync between branches all the time and is notoriously difficult to get right.

I'm not advocating to take the same approach as Solr did but it'd be nice to somehow:

* make it easier to verify that PRs put the changes entry in the right place (attach them to the right version),
* make the active-versions CHANGES.txt smaller and easier to view/convert to other formats.

Another alternative that I know of is to generate these from git logs but this requires some attention when formatting commit messages and may be hard to enforce.

[1] https://issues.apache.org/jira/browse/SOLR-17619

Review CHANGES.txt and the linked Solr issue first to understand the proposed approaches. Compare how entries are organized across active versions and how git logs might supply them. Done should include a decided workflow that verifies entry placement and makes active-version changes easier to view or convert.

Written by the indexing model from the issue text.