MAJOR: proxy: remove support for "dispatch" and "transparent" proxy keywords

MAJOR: proxy: remove support for "dispatch" and "transparent" proxy keywords

These ones were deprecated in 3.3-dev2 with commits 5c15ba5eff ("MEDIUM:
proxy: mark the "dispatch" directive as deprecated") and e93f3ea3f8
("MEDIUM: proxy: deprecate the "transparent" and "option transparent"
directives"), and were planned for removal in 3.5. See also:

   https://github.com/orgs/haproxy/discussions/2921

as well as the wiki page about breaking changes.

They've lived their lives and always cause internal limitations
(exceptions between connecting to server or connecting to proxy), and
are even confusing to some extents (especially "transparent" which users
often get wrong).

This commit removes the ability to configure them, tests based on them
and all the doc related to them. The keywords remain detected by the
parser and indicate how to proceed instead.

It's likely that other deeper parts will be changed as well (e.g.
conn->target will no longer be of OBJ_TYPE_PROXY). This will be done
over the long term.