MINOR: quic/config: adapt settings to new conn buffer limit

MINOR: quic/config: adapt settings to new conn buffer limit

QUIC MUX buffer allocation limit is now directly based on the underlying
congestion window size. previous static limit based on conn-tx-buffers
is now unused. As such, this commit adds a warning to users to prevent
that it is now obsolete.

Secondly, update max-window-size setting. It is now the main entrypoint
to limit both the maximum congestion window size and the number of QUIC
MUX allocated buffer on emission. Remove its special value '0' which was
used to automatically adjust it on now unused conn-tx-buffers.