docs: the documented install and upgrade commands do not work with 21.0.0-rc.1

[armando-navarro]

With 21.0.0-rc.1 on npm next and Angular 22 on npm latest, the commands in the Quickstart and the v21 upgrade guide no longer get anyone to a working app.

What I found

• The upgrade guide's first command fails:
  • ng update @angular/core @angular/cli targets Angular 22, and the CLI refuses the two-major jump from Angular 20: "Updating multiple major versions of '@angular/core' at once is not supported."
• The upgrade guide's second command does nothing:
  • ng update @angular/fire reports "Package '@angular/fire' is already up to date", because latest is still 20.1.0.
• The Quickstart cannot reach a working app at all:
  • Step 1 creates a project on the newest Angular, which is 22, and step 2 then installs AngularFire from latest.
  • Neither @angular/fire nor @angular/fire@next resolves against Angular 22.
• Setup stops on firebase-tools 15:
  • After the features question, ng add fails with Cannot read properties of undefined (reading 'email').
  • firebaseTools.login() returns an object with email in firebase-tools 14 and returns undefined in 15.
  • The peer range accepts 15 (firebase-tools peerDependency excludes v15 (the current major) #3701), and npm installs 15 by default.
• Thirteen pages repeat the same install command:
  • The Quickstart and twelve per-product pages each carry their own ng add @angular/fire, so every version change has to be made in thirteen places.

Scope

Documentation only. The firebase-tools 15 crash will go in its own issue/PR.