Bump @angular-devkit/build-angular in /web_embedding/ng-flutter

Bumps [@angular-devkit/build-angular](https://github.com/angular/angular-cli) from 20.3.15 to 21.1.2.
- [Release notes](https://github.com/angular/angular-cli/releases)
- [Changelog](https://github.com/angular/angular-cli/blob/main/CHANGELOG.md)
- [Commits](https://github.com/angular/angular-cli/compare/v20.3.15...v21.1.2)

---
updated-dependencies:
- dependency-name: "@angular-devkit/build-angular"
  dependency-version: 21.1.2
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

web_embedding/ng-flutter/package.json

@@ -26,7 +26,7 @@
     "zone.js": "~0.15.0"
   },
   "devDependencies": {
-    "@angular-devkit/build-angular": "^20.0.2",
+    "@angular-devkit/build-angular": "^21.1.2",
     "@angular/cli": "~21.1.1",
     "@angular/compiler-cli": "^20.0.3",
     "@types/jasmine": "~6.0.0",