Rollup merge of #131616 - RalfJung:const_ip, r=tgross35

merge const_ipv4 / const_ipv6 feature gate into 'ip' feature gate

rust-lang/rust#76205 has been closed a while ago, but there are still some functions that reference it. Those functions are all unstable *and* const-unstable. There's no good reason to use a separate feature gate for their const-stability, so this PR moves their const-stability under the same gate as their regular stability, and therefore removes the remaining references to rust-lang/rust#76205.

Author: matthiaskrgr