chore: Karma is deprecated

[lread]

Currently

We use karma to verify that rewrite-clj works in the web browser.

But

Karma has been deprecated.

So

There is no immediate need to switch to some other solution:

Critical security issues in Karma will still be triaged and fixed as necessary. This will continue until 12 months after Angular CLI's Web Test Runner support is marked stable.

But we should probably eventually jump to something supported/maintained.