meson.build: Keep the protocol version looking like xserver 1.20.x did

This effectively changes the versioning to be as if xserver 21.0 was
xserver 1.21.0. This should keep any client-side version checks that
know about the Xorg 7.0 -> xserver 1.0 epoch from getting confused.

Fixes: 1e72c3ce "Bump version to 21.0.99.1"
3 jobs for xwayland-21.1-protocol-version-backcompat in 2 minutes and 24 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #7687452
debian-buster

00:00:12

 
  Build And Test
passed #7687453
meson

00:01:39

passed #7687454
meson-noglamor

00:02:11