Deprecate BlinkMacSystemFont in favor of system-ui

[shawnbot]

This issue has a storied history, as documented by @jonathantneal in this comment. Here's the status of system-ui as of this issue's creation, as culled form caniuse.com and chromestatus.com:

Browser	Font name	Status
Chrome	system-ui	✅shipped in Chrome 56 (c. 2017)
Safari	system-ui	✅shipped in Safari 11 (c. 2017)
Firefox	-apple-system	✅shipped in Firefox 43 (c. 2015)
Firefox	system-ui	🗣 "public support" (but no intent to implement)
Edge	-apple-system	✖️No public signals
Edge	system-ui	✖️No public signals

Given the above status, I would suggest replacing BlinkMacSystemFont with system-ui, -apple-system. Edge doesn't support any of those, so it will continue to fall back on sans-serif.

@broccolini started this process in #408, but that PR is really old, and @mdo had some concerns about the appearance of some Linux system fonts. We have some long-running issues with type on Linux, so I'd like to look at what other frameworks have done before we make a final decision and move forward with the change.

[simurai]

If this comes up again, system-ui seems to have a problem with Simplified Chinese. See article and article. But that could also be outdated by now?

[github-actions]

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