meson: deprecated 'true' and 'false' in combo options for 'enabled' and 'disabled'

meson: deprecated 'true' and 'false' in combo options for 'enabled' and 'disabled'

To prepare to use meson's builtin feature options in the future, which
are more powerful and provide useful feature for packagers, like the
ability to turn all "automagic" features off, and then explicitly turn
on the ones they want.

This is designed to make the transition softer, since the 'true' and
'false' are still accepted, just with a warning.

Acked-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4740>