docs: remove references to removed utility API docs

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/auth/index.md

@@ -60,9 +60,8 @@ small update to show this provider as a login option. The `SignInPage` component
 handles this, and takes either a `provider` or `providers` (array) prop of
 `SignInProviderConfig` definitions.
 
-These reference the [ApiRef](../reference/utility-apis/README.md) exported by
-the provider. Again, an example using GitHub that can be adapted to any of the
-built-in providers:
+These reference the `ApiRef` exported by the provider. Again, an example using
+GitHub that can be adapted to any of the built-in providers:
 
 ```diff
 # packages/app/src/App.tsx

docs/auth/using-auth.md

@@ -28,7 +28,8 @@ OAuth helps in that regard.
 The method with which frontend plugins request access to third party services is
 through [Utility APIs](../api/utility-apis.md) for each service provider. For a
 full list of providers, see the
-[Utility API References](../reference/utility-apis/README.md).
+[@backstage/core-plugin-api](../reference/core-plugin-api.md#variables)
+reference.
 
 ### Identity - WIP