Auto merge of #129238 - umgefahren:stabilize-ipv6-unique-local, r=dtolnay

Stabilize `Ipv6Addr::is_unique_local` and `Ipv6Addr::is_unicast_link_local`

Make `Ipv6Addr::is_unique_local` and `Ipv6Addr::is_unicast_link_local` stable (+const).

Newly stable API:

```rust
impl Ipv6Addr {
	// Newly stable under `ipv6_is_unique_local`
	const fn is_unique_local(&self) -> bool;

	// Newly stable under `ipv6_is_unique_local`
	const fn is_unicast_link_local(&self) -> bool;
}
```

These stabilise a subset of the following tracking issue:
- #27709

I have looked and could not find any issues with `is_unique_local` and `is_unicast_link_local`. There is a well received comment calling for stabilisation of the latter function.

Both functions are well defined and consistent with implementations in other languages:
- [Go](https://cs.opensource.google/go/go/+/refs/tags/go1.23.0:src/net/netip/netip.go;l=518)
- [Python](https://github.com/python/cpython/blob/e9d1bf353c3ccafc0d9b61b1b3688051bc976604/Lib/ipaddress.py#L2319-L2321)
- [Ruby (unique local)](https://ruby-doc.org/stdlib-2.5.1/libdoc/ipaddr/rdoc/IPAddr.html#private-3F-source)
- [Ruby (unicast link local)](https://ruby-doc.org/stdlib-2.5.1/libdoc/ipaddr/rdoc/IPAddr.html#link_local-3F-source)

cc implementor `@little-dude`
(I can't find the original PR for `is_unqiue_local`)

r? libs-api
`@rustbot` label +T-libs-api +needs-fcp

Author: bors