feat: update to Angular 21 (and switch to Vitest)

[timdeschryver]

Some changes:

• wait-for fixture needed to be updated to use Signals to prevent ExpressionChangedAfterItHasBeenCheckedError
• wait-for-element-to-be-removed fixture needed to be updated to use Signals to prevent ExpressionChangedAfterItHasBeenCheckedError
• fakeAsync test needed to be removed because it requires zone.js
• defer test on click needed to use screen.findByText after a click instead of screen.getByText

Closes #524