Remove the native system store from the keyring providers (#15612)

We're not sure what the best way to expose the native store to users is
yet and it's a bit weird that you can use this in the `uv auth` commands
but can't use any of the other keyring provider options. The simplest
path forward is to just not expose it to users as a keyring provider,
and instead frame it as a preview alternative to the plaintext uv
credentials store. We can revisit the best way to expose configuration
before stabilization.

Note this pull request retains the _internal_ keyring provider
implementation — we can refactor it out later but I wanted to avoid a
bunch of churn here.

docs/concepts/authentication/cli.md

@@ -52,9 +52,7 @@ $ uv auth token --username foo example.com
 
 ## Configuring the storage backend
 
-By default, credentials are persisted in plain text to the uv
-[credentials file](./http.md#the-uv-credentials-file).
+Credentials are persisted to the uv [credentials store](./http.md#the-uv-credentials-store).
 
-If the [native keyring provider](./http.md#the-native-keyring-provider) is enabled, it will be used
-instead, and the credentials will be stored in a secure system store. The native keyring is
-currently experimental, but will become the default in the future.
+By default, credentials are written to a plaintext file. An encrypted system-native storage backend
+can be enabled with `UV_PREVIEW_FEATURES=native-auth`.