Allow FontAwesome version 6

[Jnix284]

Developers are limited to using version 5.14.0 and can't update themes to version 6.

Who: developers creating themes and marketers using themes

What/goal: ability to use the latest icon sets

Value: adds backwards compatibility

If the theme was created with 5.0.10 it is not possible to update to 5.14.0 because 5.14.0 is not downward compatible with 5.0.10.

Updating the icon_set would break the icons on existing website pages where 5.0.10 was used.

Fontawesome introduced downward compatibility with version 6, but since HubSpot doesn't allow version 6, it's not possible to update existing themes.

[LMaseraGarza]

I would love to be able to use the new X-twitter logo, but I can't because it's part of the 6.4.2 release.