databases/mysql80-{client, server}: Update to new release 8.0.15
This is a quick release after 8.0.14 which fixes Bug #29249542, Bug #94004
on upstream (Group repl. doesn't work when IPv6 is disabled on OS)
More info on upstream: https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-15.html

While here, delete the no-more-needed warning from "pkg-message" since MySQL8.0 is GA.

Sponsored by: The FreeBSD Foundation