RFC 4291 errata 4406 was rejected

[ghananigans]

Ipv6Addr::is_unique_link_local and Ipv6Addr::is_unique_link_local_strict both reference RFC 4291 errata 4406, but this errata was rejected. We should probably remove the rejected errata and just refer to the sections in the RFC for each method.

https://doc.rust-lang.org/std/net/struct.Ipv6Addr.html#method.is_unicast_link_local_strict
https://doc.rust-lang.org/std/net/struct.Ipv6Addr.html#method.is_unicast_link_local
https://www.rfc-editor.org/errata/eid4406