Add notes for [#47144][] to the 7.1 Release Notes.

Additionally, in the time since that was merged, both Nokogiri and
Minitest have merged the PRs mentioned to integrate support for Ruby's
Pattern matching (https://github.com/sparklemotion/nokogiri/pull/2523
and https://github.com/minitest/minitest/pull/936, respectively).

This commit adds coverage for those new assertions, and incorporates
guidance into the release notes.

[#47144]: https://github.com/rails/rails/pull/47144