BUG/MEDIUM: mux-h2: fix the detection of the ext connect support

BUG/MEDIUM: mux-h2: fix the detection of the ext connect support

As reported by Huangbin Zhan (@zhanhb) in github issue #3355, latest
commit 96f7ff4fdd ("MINOR: mux-h2: add a new message flag to indicate
ext connect support") was not correct and can break RFC8441-compliant
clients, as it did for them with a variant of Chrome 142.

The problem is that while RFC9113 says that new pseudo-headers are only
permitted with *negotiated* extensions, and RFC8441 doesn't indicate
whether or not SETTINGS_ENABLE_CONNECT_PROTOCOL is needed from clients,
it only says that clients know that servers support the extension when
seeing it in their settings and can use it, which seems to imply that
they don't need to send it to indicate their willingness to use it.
This also means that the server cannot know if a client is expected to
use it or not by default. It only know that a client is not allowed to
use it if the server didn't emit support mentioning it, which haproxy
can do using h2-workaround-bogus-websocket-clients.

Thus the fix proposed by @zhanhb is right, when presetting the flag for
the parser to indicate whether or not we're willing to accept RFC8441's
:protocol pseudo-header, we should:
  - consider the received setting on the backend side (though the
    pseudo-header is neither used nor supported there, but at least
    we pass the info regarding the support of the extension)
  - consider the configuration for the frontend (since it's the only
    place where we can decide on support or not)

This patch does just that and reverts the accompanying changes to the
regtests that made them want to see the client's setting. It must be
backported to 2.6.

In the mean time, placing this option in the global section will force
the clients to downgrade to h1:

    h2-workaround-bogus-websocket-clients

Many thanks again to @zhanbb this feedback and proposing a tested fix.