Bump version to 21.0.99.901

Xwayland 21.1.0 release candidate 1.
3 jobs for xwayland-21.1.0-rc1 in 3 minutes and 16 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #7380736
debian-buster

00:00:08

 
  Build And Test
passed #7380737
meson

00:03:07

passed #7380738
meson-noglamor

00:01:15