meson: Switch project version to 20.x.y style

4 jobs for xserver-21-fix-protocol-release-number in 5 minutes and 49 seconds
Status Job ID Name Coverage
  Docker Image
passed #8665099
debian-buster

00:00:17

 
  Build And Test
passed #8665100
autotools

00:05:32

passed #8665101
meson

00:01:55

passed #8665102
meson-noglamor

00:02:11