BUG/MINOR: quic: fix counters used on BE side

BUG/MINOR: quic: fix counters used on BE side

quic_conn is initialized with a pointer to its proxy counters. These
counters are then updated during the connection lifetime.

Counters pointer was incorrect for backend quic_conn, as it always
referenced frontend counters. For pure backend, no stats would be
updated. For listen instances, this resulted in incorrect stats
reporting.

Fix this by correctly set proxy counters based on the connection side.

This must be backported up to 3.3.