mysqlclient 2.2 dropped support for compiling using the mysql_config

script to locate headers/libraries, instead requiring a mysql/mariadb

pkg-config file. This flat-out breaks compatibility on many systems,

such as:

Alpine Linux (https://github.com/PyMySQL/mysqlclient/issues/620)
Amazon Linux (https://github.com/PyMySQL/mysqlclient/issues/634)
Debian Bookworm (https://github.com/PyMySQL/mysqlclient/issues/637)

This is a pretty serious issue for users, and doesn't appear to be

something the project is interested in addressing in their packaging.

Unfortunately, despite the size of this imposed breakage, there's no

compatibility lists anywhere for mysqlclient that we can reference and

provide for our own users.
Until the situation is sorted, or an alternative to mysqlclient is

found/created/forked, the best we can do is cap our default

ReviewBoard[mysql] extras package to 2.1.x, in order to preserve

existing installs, and then provide a big warning and extensive

instructions in our documentation.

Built the docs and verified the links.
Tested installing the extras package and verified I got 2.1.1.