Rollup merge of #75150 - nanpuyue:deprecate_to_ipv6_compatible, r=LukasKalbertodt
Rollup merge of #75150 - nanpuyue:deprecate_to_ipv6_compatible, r=LukasKalbertodt

Add a note for Ipv4Addr::to_ipv6_compatible

Previous discussion: #75019

> I think adding a comment saying "This isn't typically the method you want; these addresses don't typically function on modern systems. Use `to_ipv6_mapped` instead." would be a good first step, whether this method gets marked as deprecated or not.

_Originally posted by @joshtriplett in https://github.com/rust-lang/rust/pull/75150#issuecomment-680267745_

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust