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

[reins-ch]

… files

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

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• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
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What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
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[alan-agius4]

LGTM, thanks

[clydin]

This PR was merged into the repository. The changes were merged into the following branches:

• main: ff519b8
• 21.0.x: 7222b54