net-mgmt/zabbix7-server: default package settings

Hi Juraj!

I have been using Zabbix for some time now and really appreciate that it 
is now built with MySQL support enabled. The very recent change of the 
default MySQL version has shown up a slight issue: the default port 
setting for poudriere is MYSQL=ON which pulls in MySQL v8 client however 
this causes a conflict when other ports require the FreeBSD default 
version which is now 8.4 rather then 8.0.

Setting the option MYSQLDV=ON deals with this however it is not set by 
default and using it requires adding the port to the local poudriere 
build list. Not a big deal since I use poudriere for other packages 
anyway but please would you consider setting MYSQLDV=ON instead of 
MYSQL=ON so that the OS default MySQL version can be used to build the 
package?

Many thanks,

Simon Wright.