Stabilize the uv build backend (#14311)

The uv build backend has gone through some feedback cycles, we expect no
more major configuration changes, and we're ready to take the next step:
The uv build backend in stable.

This PR stabilizes:

* Using `uv_build` as build backend
* The documentation of the uv build backend
* The direct build fast path, where uv doesn't use PEP 517 if you're
using `uv_build` in a compatible version.
* `uv build --list`, which is limited to `uv_build`.

It does not:
* Make `uv_build` the default on `uv init`
* Make `--package` the default on `uv init`

docs/reference/settings.md

@@ -396,10 +396,6 @@ pydantic = { path = "/path/to/pydantic", editable = true }
 
 Settings for the uv build backend (`uv_build`).
 
-!!! note
-
-    The uv build backend is currently in preview and may change in any future release.
-
 Note that those settings only apply when using the `uv_build` backend, other build backends
 (such as hatchling) have their own configuration.