The auth guide and the sample still import beforeAuthStateChanged from firebase/auth #3815

The cookie sync in step 2 of the server-side rendering section of docs/auth.md, and sample/src/app/auth/auth.component.ts, import beforeAuthStateChanged from firebase/auth instead of @angular/fire/auth. That was a workaround for #3748: AngularFire's version kept the app marked as busy until its callback first ran, and the callback only runs when someone signs in or out.

The fix (#3770) shipped in AngularFire 20.1.1 and 21.0.0-rc.2.

So the guide and the sample can import beforeAuthStateChanged from @angular/fire/auth like the rest of the guide. The note should stay for the releases that still have the bug: 20.1.0 and earlier, and 21.0.0-rc.0 and rc.1.