Stabilize `IpAddr::is_ipv4` and `is_ipv6` as const Insta-stabilize the methods `is_ipv4` and `is_ipv6` of `std::net::IpAddr` as const, in the same way as [PR#76198](https://github.com/rust-lang/rust/pull/76198). Possible because of the recent stabilization of const control flow. Part of #76225 and #76205.