MEDIUM: proxy: deprecate the "transparent" and "option transparent" directives

MEDIUM: proxy: deprecate the "transparent" and "option transparent" directives

As discussed here [1], "transparent" (already deprecated) and
"option transparent" are horrible hacks which should really disappear
in favor of "server xxx 0.0.0.0" which doesn't rely on hackish code
path. This old feature is now deprecated in 3.3 and will disappear in
3.5, as indicated here [2]. A warning is emitted when used, explaining
how to proceed, and how to silence the warning using the global
"expose-deprecated-directives" if needed. The doc was updated to
reflect this new state.

[1] https://github.com/orgs/haproxy/discussions/2921
[2] https://github.com/haproxy/wiki/wiki/Breaking-changes