Revert font stack changes

[simurai]

This reverts the following PRs:

• Improve font rendering on Linux #1529
• Improve font stack on Windows #1573

Reasoning

We got a few reports that these changes had negative effects for certain users. We still would like to fix the initial issue #1209, but it seems not that easy and needs some more careful testing.

Next steps

An approach we might can try next is to put these font changes behind a feature flag and then enable it only for certain users that would like to beta test it (Linux, Windows, Japanese, Chinese etc.). Once we find a good font stack that works for most, we can ship it to production to all users.

[changeset-bot]

🦋 Changeset detected

Latest commit: a1ad77e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/css	Minor

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[NightFurySL2001]

Unless GitHub do either of the following things:

1. providing a panel in user settings to write their own CSS font fallback list

or

2. providing ways to tag the language of a Markdown/comment,

there will be no easy way out to write a CSS that will be good for most users without using system-ui. The benefit of listing system-ui in the first of font stack is that it will be changed based on system language (verified on Windows) and will display correctly using the system font, however all bets are off for languages not specified in system-ui which will depends on the fallback list in the system. Why? There are also Thai, Korean, Aboriginals, Arabic and more languages that currently can only use one font file to display their languages, and in the case of Chinese and Japanese they even share the same CJK Unified Ideographs but they should be displayed differently by their regions (See Source Han Sans for more info), and which one superseding the other in a fallback list will affect how the final display is shown.