[xwayland-21.1] 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"

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