Upgrade Font Awesome 4 to 6

[bennothommo]

Replaces #445 and #447.

This PR upgrades Font Awesome 4 to Font Awesome 6, bringing the number of available icons from 675 to 1748, as mentioned in the above PRs. It has been implemented using Font Awesome's LESS files, but keeps everything backwards compatible by shimming the old icon variables and ensuring the old classes point to the correct font weighting and icon.

This does increase the Backend assets from 4.84MB to 5.06MB (an increase of ~220kb) uncompressed - I haven't tested this using GZIP but would estimate it would equate to around 50-60kb, so shouldn't be too much of a hit.

[bennothommo]

Screenshots

Before

After

[LukeTowers]

@bennothommo are you also able to update the UI Icon docs in this PR? Should we like to the FontAwesome 6 site instead of hosting it all ourselves?

[bennothommo]

@LukeTowers I was thinking that, but we do use a different class structure for the icons. They provide a JSON schema of all icons so it may not be too difficult to create a component that can generate the list in our docs.

[LukeTowers]

16k icons is a lot. I'd be fine with just including instructions for how to change their class names to ours and then linking to their docs. They have a custom algolia integration in order to search all of the icons, I don't really want to go down that road on our end too.

[WebVPF]

@bennothommo there is no content property for brand icons in css.
Example: https://github.com/wintercms/winter/blob/wip/1.2/modules/system/assets/ui/icons.css#L6299-L6301

.icon-css3:before,
.wn-icon-css3:before,
.oc-icon-css3:before{font-family:'Font Awesome 6 Brands';font-weight:400}

[bennothommo]

@WebVPF sorry, it appeared I hadn't included the brand icons in the compiled icon CSS.

They're included now. :)