fix(auth): stop beforeAuthStateChanged from holding the app unstable (#3770)

AngularFire wrapped beforeAuthStateChanged so that registering the
hook added a pending task, cleared only when the callback first runs.
Firebase runs that callback only on a sign-in or sign-out, so for a
visitor who does neither the app never became stable. Registered on
the server, it failed ng build during route extraction and left
server-rendered requests without a response.

This restores the blockUntilFirst: false override from #3590, which
#3613 dropped without comment while adding log-level overrides next
to it. The callback still runs inside Angular's zone and injection
context, and its returned promise still reaches Firebase, so a
rejection still cancels the sign-in. A call outside an injection
context now logs its per-call warning only at the verbose level, as
onMessage does.

Fixes #3748

docs(auth): scope the beforeAuthStateChanged note to rc.1 and earlier

Merging this change closes #3748, so the section's present-tense note
would point at a closed issue. Also removed the false claim that the
@angular/fire/auth import makes ng build hang: the guide registers the
hook only in the browser, so its own build succeeds.

The docs/auth.md change is left out, since it edits a section of the
guide that 20.1.x does not have.

(cherry picked from commit f182972)

Author: armando-navarro

src/auth/firebase.ts

@@ -139,6 +139,8 @@ ${exportedZoneWrappedFns}
       indexedDBLocalPersistence: null,
       prodErrorMap: null,
       multiFactor: null,
+      // Its callback fires only on a sign-in or sign-out, so blocking would keep `ApplicationRef.isStable` false.
+      beforeAuthStateChanged: { blockUntilFirst: false },
       linkWithCredential: { logLevel: LogLevel.VERBOSE },
       linkWithPhoneNumber: { logLevel: LogLevel.VERBOSE },
       linkWithPopup: { logLevel: LogLevel.VERBOSE },