BUG/MEDIUM: h3: ensure the ":method" pseudo header is totally valid

BUG/MEDIUM: h3: ensure the ":method" pseudo header is totally valid

Ensure pseudo-header method is only constitued of valid characters
according to RFC 9110. If an invalid value is found, the request is
rejected and stream is resetted.

Previously only characters forbidden in headers were rejected (NUL/CR/LF),
but this is insufficient for :method, where some other forbidden chars
might be used to trick a non-compliant backend server into seeing a
different path from the one seen by haproxy. Note that header injection
is not possible though.

This must be backported up to 2.6.

Many thanks to Yuki Mogi of FFRI Security Inc for the detailed report
that allowed to quicky spot, confirm and fix the problem.