[UFO][WEAPON][OXCE] Replace Heavy Cannon with RT-20

[Zeta Reticulan]

The Croatian RT-20 "Hand Cannon" is a bullpup, 20mm anti-material rifle. The tube mounted above the barrel redirects propellent gas backwards to counteract the recoil, similar to that of a recoilless rifle. The RT-20 is a bolt action, single shot weapon that needs to be reloaded after every shot. It's heavy and cumbersome, but highly accurate and packs a powerful punch.

https://mod.io/g/openxcom/m/rt-20