Update supported browser versions

What does this MR do and why?

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

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