MINOR: quic: Replace max_packet_size by max_udp_payload size.

The name the maximum packet size transport parameter was ambiguous and replaced
by maximum UDP payload size. Our code would be also ambiguous if it does not
reflect this change.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index d926b82..327c3ce 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -50,9 +50,9 @@
 #include <haproxy/xprt_quic.h>
 
 struct quic_transport_params quic_dflt_transport_params = {
-	.max_packet_size    = QUIC_DFLT_MAX_PACKET_SIZE,
-	.ack_delay_exponent = QUIC_DFLT_ACK_DELAY_COMPONENT,
-	.max_ack_delay      = QUIC_DFLT_MAX_ACK_DELAY,
+	.max_udp_payload_size = QUIC_DFLT_MAX_UDP_PAYLOAD_SIZE,
+	.ack_delay_exponent   = QUIC_DFLT_ACK_DELAY_COMPONENT,
+	.max_ack_delay        = QUIC_DFLT_MAX_ACK_DELAY,
 };
 
 /* trace source and events */