x11-toolkits/wxgtk28: mark deprecated

wxgtk28 is not maintained upstream. All ports still using wxgtk28:

• Are abandonned upstream
• Can be built with wxgtk 3.2 (only games/blokish requires 3.0)
• Are broken (emulators/lisaem)

Approved by: portmgr
Differential Revision: https://reviews.freebsd.org/D37129