Add doc deprecation recommending @web/test-runner-playwright

[trivikr]

Is your feature request related to a problem? Please describe.

karma has been deprecated https://github.com/karma-runner/karma/pull/3846

For browser-based unit testing solutions, karma recommends Web Test Runner and jasmine-browser-runner.

There's no such deprecation notice in karma-firefox-launcher which is an extension for karma

Describe the solution you'd like

Add doc deprecation in README, recommending readers to try @web/test-runner-playwright instead.

Browser Launchers for Web Test Runner: https://modern-web.dev/docs/test-runner/browser-launchers/overview/

Describe alternatives you've considered

N/A

[karmarunnerbot]

:tada: This issue has been resolved in version 2.1.3 :tada:

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

Your semantic-release bot :package::rocket: