Bump @angular/platform-browser in /web_embedding/ng-flutter

Bumps [@angular/platform-browser](https://github.com/angular/angular/tree/HEAD/packages/platform-browser) from 20.3.16 to 21.1.3.
- [Release notes](https://github.com/angular/angular/releases)
- [Changelog](https://github.com/angular/angular/blob/main/CHANGELOG.md)
- [Commits](https://github.com/angular/angular/commits/v21.1.3/packages/platform-browser)

---
updated-dependencies:
- dependency-name: "@angular/platform-browser"
  dependency-version: 21.1.3
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

web_embedding/ng-flutter/package.json

@@ -18,7 +18,7 @@
     "@angular/core": "^20.0.3",
     "@angular/forms": "^21.1.0",
     "@angular/material": "^21.1.1",
-    "@angular/platform-browser": "^20.0.3",
+    "@angular/platform-browser": "^21.1.3",
     "@angular/platform-browser-dynamic": "^21.1.2",
     "@angular/router": "^21.1.0",
     "rxjs": "~7.8.1",