MINOR: mux-quic/h3: prepare CONNECTION_CLOSE on release

MINOR: mux-quic/h3: prepare CONNECTION_CLOSE on release

When MUX is released, a CONNECTION_CLOSE frame should be emitted. This
will ensure that the client does not use anymore a half-dead connection.

App protocol layer is responsible to provide the error code via release
callback. For HTTP/3 NO_ERROR is used as specified in RFC 9114. If no
release callback is provided, generic QUIC NO_ERROR code is used. Note
that a graceful shutdown is used : quic_conn must emit CONNECTION_CLOSE
frame when possible. This will be provided in another patch.

This change should limit the risk of browsers stuck on webpage loading
if MUX has been released. On CONNECTION_CLOSE reception, the client will
reopen a new QUIC connection.