fix: Only use IPv4 address without dual mode

[imerr]

Previously, if the DNS resolve returned an IPv6 address as the first entry it would use that Notable for connecting to "localhost" which resolves to ::1 (ipv6) first on windows

[miwarnec]

checking the KcpClient.Connect().
it uses the addresses[0].AddressFamily:

in theory, it should make connect work even if resolve gives ipv4 or ipv6.
what are we missing here?

[imerr]

Even though you connect to the right address family, the server might not be listening on that address family

For example the case of connecting to "localhost" with dual mode off
Start server, that listens to 127.0.0.1 only (not ::1)
Client resolves localhost to ["::1", "127.0.0.1"]
Attempts to connect to "::1" since that is the first address returned
Connection fails since there's no socket listening for ipv6, only ipv4