docs: mention Vitest instead of Karma in README template of workspace files

Since version Angular 21, Vitest is the default test runner.

(cherry picked from commit ff519b8)

Author: reins-ch

packages/schematics/angular/workspace/files/README.md.template

@@ -38,7 +38,7 @@ This will compile your project and store the build artifacts in the `dist/` dire
 
 ## Running unit tests
 
-To execute unit tests with the [Karma](https://karma-runner.github.io) test runner, use the following command:
+To execute unit tests with the [Vitest](https://vitest.dev/) test runner, use the following command:
 
 ```bash
 ng test