GeezerVison

Fixes #3812

Prepares the docs on 21.0.x for the 21.0.0 tag. Four files change.

Changes

• README.md and step 2 of docs/install-and-setup.md: install with ng add @angular/fire. The quickstart's "currently a release candidate" sentence is gone.
• docs/install-and-setup.md and docs/install-firebase-tools.md: the "run ng add yourself, not through an AI agent" note is removed. It described the firebase-tools 15.26 crash from ng add @angular/fire stops with Cannot read properties of undefined (reading 'email') when firebase-tools 15.26+ detects an AI agent #3768, and the fix (fix(schematics): read the signed-in Firebase account on firebase-tools 15.26+ #3769) is on this branch.
• docs/version-21-upgrade.md: titled "Upgrading to AngularFire 21". The command is ng update @angular/core@21 @angular/cli@21 @angular/fire.
  • Only the first @angular/ package in the command picks the CLI that runs the update (docs: cover ng update stopping on a Node.js version message #3788), so @angular/fire needs no version there.
  • The Node.js note now says that updating AngularFire on its own needs ng update @angular/fire@21, not plain ng update @angular/fire.
  • The App Check note's qualifier becomes "(since 21.0.0-rc.2)", so readers coming from rc.0 or rc.1 still see that it changed between release candidates.

Left as they are: the "AngularFire 21.0.0-rc.1 and earlier" section of docs/app-check.md, which is a how-to for those releases, and the docs/auth.md note on importing beforeAuthStateChanged from firebase/auth, which is a separate change.

Merge order

Merge before 21.0.0 is tagged, so the tag carries these docs. Between the merge and the tag, the plain ng add @angular/fire in these docs does not install 21 on an Angular 21 app, since latest is 20.1.1 until the tag publishes.

Verification

Documentation only. git grep -i -E "@next|release candidate|-rc\.[0-9]" over docs/ and README.md on the branch returns the two sections listed above as left in place and the App Check note's "(since 21.0.0-rc.2)" qualifier, nothing else. The install and update commands cannot be run against 21.0.0 until it is on latest.