`system-ui` is provided by modern browsers as [a cross-platform default font](https://caniuse.com/font-family-system-ui). Using this font [was a controversial choice back in 2017](https://infinnie.github.io/blog/2017/systemui.html). because it did not address i18n well and therefore it was removed from Bootstrap [twbs/bootstrap#22377](https://github.com/twbs/bootstrap/pull/22377). However, it was added back in Bootstrap v5 [twbs/bootstrap#30561](https://github.com/twbs/bootstrap/pull/30561) since it got better over time.
Fixes T15049.