Links against libmysqlclient.so.21, not libmariadb.so.3

[Rüdiger Kuhlmann]

This perl package specifically is for accessing MariaDB. However, it's .so file links against libmysqlclient (in Ubuntu 20: libmysqlclient.so.21), and not against libmariadb.so.3, and thus depends on libmysqlclient21 (and not libmariadb3). Note that this is correct in Debian (as you can see here: https://packages.debian.org/buster/libdbd-mariadb-perl - the same is true for all other linked releases of Debian).

This can be fixed by changing the Build-Depends (from default-libmysqlclient-dev (>= 1.0.5) to libmariadb-dev-compat, and from default-mysql-server to mariadb-server).