Remove API key authentication

As stated in [https://github.com/getsentry/sentry-cli/issues/2872](https://github.com/getsentry/sentry-cli/issues/2872) ([CLI-198](https://linear.app/getsentry/issue/CLI-198/deprecate-api-key-authentication)), API key authentication has been deprecated for a very long time on the server-side, so it is probably time to remove it in Sentry CLI