[armando-navarro]

Fixes #3812

The same change as #3813, which targets 21.0.x, applied to main. The upgrade guide that the v21 announcements link is blob/main/docs/version-21-upgrade.md, so main's copy has to describe the release too. Four files change, the patch is identical to #3813's.

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) has been on main since rc.2.
• 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.

The README and quickstart on main still describe an Angular 21 project. Their move to 22 is a separate change.

Merge order

After #3813. This PR carries Fixes #3812 because GitHub only closes a linked issue when the pull request merges into the default branch, so #3813's own Fixes line does nothing.

Verification

Documentation only. The staged patch matches #3813's commit ca70d4c line for line (compared with diff over both patches, ignoring blob ids). The same git grep for release-candidate wording leaves the two sections #3813 lists and the App Check qualifier, nothing else.

[tyler-reitz]

Approving. One timing ask: please hold the merge until 21.0.0 is on latest. Right now latest is still 20.1.1, so once this lands, the README's ng add @angular/fire would install 20 on an Angular 21 app. This is the repo front page, so the gap shows up more here than on 21.0.x.