BUILD: quic: 32-bits compilation issue with %zu in quic_rx_pkts_del()

BUILD: quic: 32-bits compilation issue with %zu in quic_rx_pkts_del()

This issue arrived with this commit:
    1dbeb35f8 MINOR: quic: Add new traces about by connection RX buffer handling

and revealed by the GH CI as follows:

   src/quic_conn.c: In function â\80\98quic_rx_pkts_delâ\80\99:
    include/haproxy/trace.h:134:65: error: format â\80\98%zuâ\80\99 expects argument of type â\80\98size_tâ\80\99,
    but argument 6 has type â\80\98uint64_tâ\80\99 {aka â\80\98long long unsigned intâ\80\99} [-Werror=format=]
    _msg_len = snprintf(_msg, sizeof(_msg), (fmt), ##args);

Replace all %zu printf integer format by %llu.

Must be backported to 2.7 where the previous is supposed to be backported.