net-mgmt/zabbix6-server: A don't override DEFAULT_VERSIONS= option

Some DIY poudriere users prefer to specify $DEFAULT_VERSIONS in their
make.conf. Such users should be aware that Zabbix 6 supports only
mysql 8.0 and mariadb 10.5 and 10.6. (Though some have made it to work
with mysql 5.7 when they ignore the warning message.) Provide these users
with an option to remove the default allowing them to "responsibly"
specify the version in their poudriere make.conf.

Reported by: otis, egresh@egresh.sk
Tested by: egresh@egresh.sk