build: Bump Wayland dependency to 1.21

Merged Olivier Fourdan requested to merge ofourdan/xserver:bump-req-wayland-1.21 into master

Wayland 1.21 was released on June 30, 2022 and our CI already installs Wayland 1.21 so let's just require the version we actually use in our CI.

And with this, we do not need to build wl_pointer.axis_v120 conditionally anymore.

/cc @whot

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