Update font stack to use 'system-ui'

[janfaracik]

This MR updates the default font stack for Jenkins so that it uses 'system-ui' (https://caniuse.com/font-family-system-ui), this brings better font consistency across browsers and makes Chrome render fonts the same way as other browsers, so Chrome shouldn't over-bold fonts anymore. The character spacing has also been tightened slightly for better legibility.

Safari - Pretty much the same
Before

After

Chrome - Big difference!
Before

After

Firefox - Pretty much the same
Before

After

Proposed changelog entries

• Update Jenkins to use default system font on Chrome

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change) and are in the imperative mood. Examples
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Desired reviewers

@jenkinsci/sig-ux

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
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[daniel-beck]

From https://developer.mozilla.org/en-US/docs/Web/CSS/letter-spacing

Legible letter-spacing must be determined on a case-by-case basis, as different font families have different character widths. There is no one value that can ensure all font families automatically maintain their legibility.

Is that not a concern here? If we're just using whatever is the system font, won't that mean we have no way of knowing whether messing with letter spacing will work out?

IIRC Windows 98 let me choose crazy fonts as system fonts, is that no longer possible?

[janfaracik]

The decrease in letter spacing is extremely minimal so it shouldn't make any system font illegible, happy to undo the change though if need be.