Generate dhparam with turris-cagen instead of using separate dhparam

This changes how dhparam is generated. Until now separate script and package was used to generate dhparam while turris-cagen is able to do the same.

This is simple change and allows us to remove one dependency. It takes some time to generate CA and generating dhparam as part of that makes sense as well. It is true that it prolongs initial generation (and that was probably original reason why moving it to separate package) but this way it is more consistent. It is generated only by user and all is stored in same location.

Existing configuration is going to be migrated by fix package. That means missing dhparam is going to be generated and configuration updated with openvpn reload after that.

In reference to https://gitlab.nic.cz/turris/turris-os-packages/-/issues/505