Upgrade FA to 6.6 - Fix `downloadable font: Glyph bbox was incorrect `

[erikn69]

For those who don't want to see the annoying warning message, replace the css with 6.6 version

I don't know if this can be upgraded, but i did add css/v4-shims.min.css for backward compatibility

• Fix on FA v6 https://github.com/FortAwesome/Font-Awesome/issues/19925
• Closes https://github.com/barryvdh/laravel-debugbar/issues/1645