Bump font-awesome-sass from 5.15.1 to 6.4.0

[javierm]

References

• Depends on #5445 (or, at least, the part of that pull request where we move to Dart Sass)
• Font Awesome Sass releases
• Compare view
• Replaces #5133
• Font Awesome 6.4.0 Free zip file

Objectives

• Use Font Awesome 6.x icons
• Simplify the code to render Font Awesome icons

Notes

:warning: Looks like Font Awesome Version 6 no longer supports any version of IE. Should we postpone including these changes? If so, till when?

[javierm]

Since this update causes the icons to stop working in Internet Explorer 11, we're postponing this one for now. We'll reconsider once we upgrade to Foundation 6.8.1, since right now we're silencing the Dart Sass warnings because of Foundation and Font Awesome, and once we update both of them we'll be able to stop silencing the warnings.