auth: simplify types and reintroduce deprecated BackstageIdentity

Co-authored-by: Fredrik Adelöw <freben@gmail.com>
Co-authored-by: blam <ben@blam.sh>
Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/core-plugin-api/api-report.md

@@ -237,7 +237,7 @@ export type AuthRequestOptions = {
   instantPopup?: boolean;
 };
 
-// @public @deprecated (undocumented)
+// @public @deprecated
 export type BackstageIdentity = BackstageIdentityResponse;
 
 // @public

packages/core-plugin-api/src/apis/definitions/auth.ts

@@ -212,6 +212,8 @@ export type BackstageIdentityResponse = {
 };
 
 /**
+ * The old exported symbol for {@link BackstageIdentityResponse}.
+ *
  * @public
  * @deprecated use {@link BackstageIdentityResponse} instead.
  */