Added cross-platform default fonts

Summary:
system-ui is provided by modern browsers as a cross-platform default font. Using this font was a controversial choice back in 2017. because it did not address i18n well and therefore it was removed from Bootstrap twbs/bootstrap#22377. However, it was added back in Bootstrap v5 twbs/bootstrap#30561 since it got better over time.

Fixes T15049.

Test Plan: Tested locally for now.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: Matthew, avivey, Cigaryno, 20after4, Ekubischta, speck, tobiaswiese, valerio.bozzolan

Maniphest Tasks: T15049

Differential Revision: https://we.phorge.it/D25021