CLEANUP: quic: use QUIC_PACKET_PN_MAXLEN instead of the magic 4

CLEANUP: quic: use QUIC_PACKET_PN_MAXLEN instead of the magic 4

Both fast retransmission functions account for the packet number increase
when comparing the size of the packets to be retransmitted with the
anti-amplification limit. They use a plain 4 for this, while
quic_conn-t.h defines QUIC_PACKET_PN_MAXLEN for this exact purpose, as
already used in quic_tx.c for the header protection sample.

Replace the magic value in qc_prep_fast_retrans() and
qc_prep_hdshk_fast_retrans().

No functional change, no need to backport.