The docs on 21.0.x still describe AngularFire 21 as a release candidate #3812

With 21.0.0 about to be tagged from 21.0.x, the docs on that branch still describe the release candidate:

• README.md and step 2 of docs/install-and-setup.md install with ng add @angular/fire@next, and the quickstart says AngularFire 21 "is currently a release candidate".
• docs/version-21-upgrade.md is titled "Upgrading to the AngularFire 21 Release Candidate", its command ends in @angular/fire@next, and its Node.js note gives ng update @angular/fire@21.0.0-rc.2 as the example.
• The quickstart and docs/install-firebase-tools.md still carry the "run ng add yourself, not through an AI agent" note added for ng add @angular/fire stops with Cannot read properties of undefined (reading 'email') when firebase-tools 15.26+ detects an AI agent #3768. The fix (fix(schematics): read the signed-in Firebase account on firebase-tools 15.26+ #3769) is on 21.0.x, so every 21.0.0 install has it.

Once 21.0.0 is on npm latest, the plain ng add @angular/fire and ng update commands install it, and the 21.0.0 tag would carry the release-candidate wording permanently. The branch's docs should describe the release before it is tagged.

Scope: documentation on 21.0.x only. The "21.0.0-rc.1 and earlier" section of docs/app-check.md stays, since it is a how-to for those releases. The note in docs/auth.md about importing beforeAuthStateChanged from firebase/auth is a separate change, because switching the sample back to @angular/fire/auth needs the #3770 fix verified in an app first.