Meson: Bump minimum meson requirement to 0.57.0 for Qt6 module
CI currently does meson build with older version. That needs to be resolved before this patch can go forward.
One option is to simply wait that CI can be switched to use ubuntu-22.04 (-> #43598)
Will need rebasing when it's time to apply this.
This will also enable #42935
Qt6 module is available since meson version 0.57.0. Make that the minimum requirement, so we can add Qt6 support (in a separate ticket)
As part of #43200, this has also been posted to http://forum.freeciv.org/f/viewtopic.php?f=14&t=92096