BUG/MINOR: h2: reject extended connect for h2c protocol

BUG/MINOR: h2: reject extended connect for h2c protocol

This commit prevents forwarding of an HTTP/2 Extended CONNECT when "h2c"
or "h2" token is set as targetted protocol. Contrary to the previous
commit which deals with HTTP/1 mux, this time the request is rejected
and a RESET_STREAM is reported to the client.

This must be backported up to 2.4 after a period of observation.

(cherry picked from commit 7a5a30d28a3ee100c62603f7212cc1b313c53311)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>