Tracking issue for release notes of #127921: Stabilize unsafe extern blocks (RFC 3484)

[rustbot]

This issue tracks the release notes text for #127921.

• Issue is nominated for the responsible team (and T-release nomination is removed).
• Proposed text is drafted by team responsible for underlying change.
• Issue is nominated for release team review of clarity for wider audience.
• Release team includes text in release notes/blog posts.

Release notes text:

The section title will be de-duplicated by the release team with other release notes issues.
Prefer to use the standard titles from previous releases.
More than one section can be included if needed.

# Language
- [Stabilize unsafe extern blocks (RFC 3484)](https://github.com/rust-lang/rust/pull/127921)

Release blog section (if any, leave blank if no section is expected):

### Unsafe extern

Rust code can use functions and statics from foreign code.  The type signatures of these foreign items are provided in `extern` blocks.  Historically, all items within `extern` blocks have been unsafe to call, but we didn't have to write `unsafe` anywhere on the `extern` block itself.

However, if a signature within the `extern` block is incorrect, then using that item will result in undefined behavior.  Would that be the fault of the person who wrote the `extern` block, or the person who used that item?

We've decided that it's the responsibility of the person writing the `extern` block to ensure that all signatures contained within it are correct, and so we now allow writing `unsafe extern`:

```rust
unsafe extern {
    pub safe fn sqrt(x: f64) -> f64;
    pub unsafe fn strlen(p: *const u8) -> usize;
}
```

One benefit of this is that items within an `unsafe extern` block can be marked as safe to call.  In the above example, we can call `sqrt` without using `unsafe`.  Items that aren't marked with either `safe` or `unsafe` are conservatively assumed to be `unsafe`.

In future releases, we'll be encouraging the use of `unsafe extern` with lints.  Starting in Rust 2024, using `unsafe extern` will be required.

For further details, see [RFC 3484](https://github.com/rust-lang/rfcs/blob/master/text/3484-unsafe-extern-blocks.md) and the ["Unsafe extern blocks"](https://doc.rust-lang.org/nightly/edition-guide/rust-2024/unsafe-extern.html) chapter of the edition guide.

[traviscross]

cc @Lokathor @rust-lang/lang