Increase maximum packet size from 1024 to 1248 octets

Increase the maximum packet size from 1024 to 1248 octets to allow sending QUIC initial packets over both IPv4/UDP and IPv6/UDP.

RFC 9000 states that:

A client MUST expand the payload of all UDP datagrams carrying Initial packets to at least the smallest allowed maximum datagram size of 1200 bytes

Therefore, the smallest packet that can send a QUIC Initial datagram for IPv4/UDP and IPv6/UDP is 1248 octets; 40 octets for the IPv6 header plus 8 octets for the UDP header plus 1200 octets for the QUIC payload.

This is needed to support adding QUIC/UDP (#1791).