DOC: configuration: remove obsolete mentions of H2 being converted to HTTP/1.x

DOC: configuration: remove obsolete mentions of H2 being converted to HTTP/1.x

The first H2 implementation in version 1.8 used to turn HTTP/2 requests
to HTTP/1.1, causing many limitations. This is not true anymore and we
don't suffer from the lack of server-side H2 nor are we forced to close
mode anymore, so let's remove such obsolete mentions.

This could be backported to 2.0.

(cherry picked from commit 253c2519c2d3e394cc71630331dd79ceb14c57ec)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>