docs: correct the install and upgrade commands for 21.0.0-rc.1

[armando-navarro]

Fixes #3764

The Quickstart and the v21 upgrade guide documented commands that cannot produce a working app now that 21.0.0-rc.1 is on next and Angular 22 is on latest.

What changed

• The upgrade guide has been updated to correctly install AngularFire 21.0.0-rc.1 with Angular 21: ng update @angular/core@21 @angular/cli@21 @angular/fire@next.
• The Quickstart doc creates the project on Angular 21 and installs @angular/fire@next.
  • The bullet advising @next on the newest Angular major is gone, since the release candidate does not support Angular 22.
• The twelve per-product pages point at the Quickstart instead of each repeating ng add @angular/fire.
• The Quickstart doc and the firebase-tools page tell readers to install firebase-tools 14 when ng add stops with Cannot read properties of undefined (reading 'email').
• The README headline command installs the release candidate.

Verification

Run against a real app:

• From an Angular 20 app with AngularFire 20.1.0, firebase 11.10.0 and a getVertexAI call the upgrade guide's command:
  • produced Angular 21.2.23
  • recorded @angular/fire ^21.0.0-rc.1, which npm currently resolves to the canary 21.0.0-rc.1-canary.95b3de1 (tracked separately, see Canary builds are named after the release candidate, so npm ranks them above it #3766)
  • produced firebase ^12.18.0
  • rewrote the call to getAI(..., { backend: new AgentPlatformBackend() })
  • was confirmed to result in an app that type-checked and built
• The Quickstart's project-creation commands were each run and all produce an @angular/core ^21.2.0 app:
  • npm create @angular@21
  • yarn create @angular@21
  • ng new from @angular/cli@21
• ng add @angular/fire@next was run in a fresh Angular 21 app, selecting an existing Firebase project and web app.
  • It wrote provideFirebaseApp and provideAuth into app.config.ts, and the app built.
• The firebase-tools 15 crash was reproduced:
  • Calling login() directly returns undefined on 15.28.1 and an object carrying email on 14.27.0.

[tyler-reitz]

Requesting changes on one point; the rest is non-blocking.

The firebase-tools guidance contradicts itself, and a clean machine still breaks. Three install commands are unpinned and resolve to 15.30.2 today, the version this PR says fails:

• docs/install-and-setup.md:5 tells readers ng add installs the CLI for them, so they do not have to act first. But src/schematics/firebaseTools.ts:30 runs npm i --location=global firebase-tools unpinned, so on a machine with no CLI, ng add installs 15 itself and hits the crash every time. The bullet's own remedy, npm install -g firebase-tools, also lands on 15.
• docs/install-firebase-tools.md: the new banner says install 14, and every command below it installs latest.

Pinning @14 in those commands and reversing the first bullet closes it.

Non-blocking:

• docs/compat.md:4 carries the same headline command that was updated in the README.
• site/src/get-started/quick-start.md:21 still has the bare command and is the /get-started redirect target. Pre-existing, so fine to leave, but it is the last page with this defect.
• #3764 says the firebase-tools 15 crash gets its own issue. I do not see one open. The unguarded reads are setup/prompts.ts:110 (defaultUser.email) and setup/index.ts:95 (user.email, where userPrompt can return login()'s undefined).

I did not run ng add against firebase-tools 15; that part is read from the call sites. The unpinned install and latest = 15.30.2 I checked.

[tyler-reitz]

Narrowing my review: I am dropping the @14 pinning ask, it can ride with the crash fix PR.

One thing left before I approve. docs/install-and-setup.md:5 still says ng add installs the CLI if it is missing, "so you do not have to install it yourself first". That is wrong today: the auto-install is unpinned (src/schematics/firebaseTools.ts:30), so a reader with no CLI gets 15 and hits the crash every time. Rewording that bullet is a docs fix and belongs here regardless of who fixes the crash.

Could you also file the firebase-tools 15 issue #3764 promised? Nothing is open for it yet.

[armando-navarro]

@tyler-reitz Thanks for the review. Both asks are implemented:

1. The Firebase CLI bullet now says to install version 14 first, and says what happens on 15 and on anything older than 14. You are right that the auto-install is what makes this bite a clean machine: firebaseTools.ts:30 installs the newest version, which is 15.30.2 today.

2. The crash has its own issue now, ng add @angular/fire stops with Cannot read properties of undefined (reading 'email') on firebase-tools 15 #3768, with the two unguarded reads you identified. I reproduced it end to end: login() returns an object with email on 14.27.0 and undefined on 15.28.1, called under a terminal.

Leaving docs/compat.md and the site/ page as they are, per your non-blocking note.