Update supported browser versions

This increases supported browser versions to:

- Mozilla Firefox: at least major version 91, as this is the latest ESR
  and it is still used quite a lot.
- Google Chrome/Chromium/Microsoft Edge: at least major version 92
  because it is about the same age as Firefox ESR 91
- Apple Safari: at least version 14.1 (dropping support for
  Safari 13 and 13.1) because it is the newest minor in the previous
  major version of Safari

This is still more permissive than what we strive to support, but still
strikes a good balance:

https://docs.gitlab.com/ee/install/requirements.html#supported-web-browsers

Changelog: changed