[request-tracker-maintainers] Bug#986707: rt4-db-mysql: should declare an incompatibility with mysql 8

[Dominic Hargreaves]

Package: rt4-db-mysql
Version: 4.4.4+dfsg-2

request-tracker4 as shipped with Ubuntu 20.04 is broken when used with
the MySQL release also shipped in Ubuntu. Whilst we can't fix that
directly in Debian, we an arrange for the dependencies to correctly 
reflect this, maybe by conflicting with mysql-server-8.0, and/orrequiring
mariadb explicitly.

Ubuntu bug report: https://bugs.launchpad.net/ubuntu/+source/request-tracker4/+bug/1923264