protocol: deprecate non-current wl_output.mode

The current wl_output.mode event has several issues when used to advertise
modes that aren't current:

- It's not possible to remove some modes. This is an issue for virtual
  outputs and when the kernel prunes some modes because of link limitations.
- wl_output.mode fails to carry metadata such as aspect ratio, which results
  in duplicated or missing modes.
- It's not clear, given the current set of protocols, how non-current modes
  are useful to clients. Xwayland ignores non-current modes.

GNOME and wlroots already only advertise the current mode because of these
issues.

If a protocol needs the clients to know about all available modes, it should
advertise these modes itself instead of relying on wl_output.mode.

Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: https://gitlab.freedesktop.org/wayland/wayland/-/issues/92