Enabling changelog verifier

Hi everyone,

We've been working on a bot to look at PRs, check for a CHANGES entry (
#13898 <https://github.com/apache/lucene/issues/13898>), and determine the
milestone the PR belongs to (#14190
<https://github.com/apache/lucene/issues/14190>).

I got a few approvals to enable this bot in #14644
<https://github.com/apache/lucene/pull/14644>, but I want to tell you all
what to expect. If you want to see an example, look at the PR I've been
testing on <https://github.com/stefanvodita/lucene/pull/9>.
1. If a PR does not have a CHANGES entry, the bot should post a comment
reminding us to create one.
2. If a PR has a CHANGES entry, the bot will try to find the release that
entry is under and add the corresponding milestone to the PR.

I'll keep an eye out on the bot, but if this isn't the behaviour you're
seeing, it would be useful to know! It won't work perfectly from the start,
but I think at this stage the bot should be able to handle most PRs
correctly. I'm planning to enable the bot tomorrow.

Stefan