fix(core): drop vestigial @angular/platform-browser-dynamic peer (#3718)

Every platform-browser-dynamic 20.x/21.x patch release requires the exact @angular/core and @angular/common version matching its own patch. Declaring pbd as a required peer therefore makes npm pull in a pbd version whose required core/common usually differs from the app's installed core/common, failing `ng add @angular/fire` with ERESOLVE on Angular >= 20.1 apps.

Nothing in the published package imports platform-browser-dynamic; the only usage in the repo is the Karma test bootstrap (src/test.ts), which is supplied by the root package.json and is unaffected. Removing a peer nothing imports is non-breaking for consumers.

Considered marking the peer optional via peerDependenciesMeta instead (the manifest already uses that pattern for firebase-tools and platform-server); it would also avoid the ERESOLVE on npm, but it keeps advertising a dependency relationship that does not exist and relies on npm-specific resolver behavior. Deletion fixes every package manager.

Refs #3667

Author: armando-navarro

src/package.json

@@ -26,7 +26,6 @@
     "@angular/common": "^21.0.0",
     "@angular/core": "^21.0.0",
     "@angular/platform-browser": "^21.0.0",
-    "@angular/platform-browser-dynamic": "^21.0.0",
     "@angular/platform-server": "^21.0.0",
     "rxjs": "~7.8.0",
     "firebase-tools": "^14.0.0 || ^15.0.0"