build: require Angular 21.2 and compile with its toolchain (#3763)

Angular 21 entered long-term support on 2026-06-03, and long-term support
ships only critical fixes and security patches, so 21.2 is the last 21.x
minor there will be. The raise is release coordination for the
TransferState work (#3757), whose signal-based reading functions build on
resourceFromSnapshots, absent from @angular/core 21.0 and 21.1 and
present in 21.2.

ng-packagr 21.2 defaults the built package.json to "type": "module",
which makes Node read the CommonJS schematics bundles as ES modules and
breaks ng add and ng deploy. src/package.json now declares
"type": "commonjs", which ng-packagr's ??= default leaves alone.

From review: replacePackageCoreVersion now fails when its placeholder is
not found, the schematics-load check fails an entry point that loads
while exporting nothing, and docs/version-21-upgrade.md no longer states
the old ^21.0.0 peer range, which shipped inside the package.

Author: armando-navarro

docs/version-21-upgrade.md

@@ -79,5 +79,5 @@ Two symbols were removed rather than renamed. The migration leaves their imports
 
 ## Other notes
 
-- **Angular 21 is required.** AngularFire 21 peers `@angular/* ^21.0.0` and does not support Angular 22 (a future AngularFire 22 will).
+- **Angular 21.2 is required.** AngularFire 21 peers `@angular/* ^21.2.0` and does not support Angular 22 (a future AngularFire 22 will).
 - The obsolete `@angular/platform-browser-dynamic` peer dependency was removed. No action is needed.