Support SF Mono font in Firefox and Safari (take 2)

[simurai]

This is another attempt after reverting #992 (comment).

I paired with @nickcanz (thanks 🙇 ) and we found the following:

• This stackoverflow is correct and an older version of SF Mono causes the "bold" problem. Updating to a new version fixed it.
• For users that still have the old version installed, having SFMono-Regular declared before SF Mono also fixes the problem.

With that we have the following stack:

$mono-font:
  ui-monospace, // support for Safari
  SFMono-Regular, // support for Chrome with an older version of SF Mono
  SF Mono, // support for Firefox
  Consolas, Liberation Mono, Menlo, monospace !default;

Also removed the " since it gets stripped anyways when compiling the SCSS.

[changeset-bot]

🦋 Changeset detected

Latest commit: 91f18ab

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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[jonrohan]

Worth a second chance, we should be ready with a patch release if we need to revert this quickly.