feat: update Font Awesome from v5.15.3 to v6

[rodrigoalcarazdelaosa]

Purpose

This PR updates Font Awesome to its latest version, following instructions found here, addressing #2649. v6 is fully compatible with v5, so no issues are expected. The yarn.lock file has also been updated installing the package via npm.

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[rodrigoalcarazdelaosa]

@gcushen if we're just using free icons perhaps we could take the opportunity to select just the CSS files for the styles we need to cut down on file size and improve performance, as suggested here.

[gcushen]

@gcushen if we're just using free icons perhaps we could take the opportunity to select just the CSS files for the styles we need to cut down on file size and improve performance, as suggested here.

I don't know how big the performance benefit is of including FontAwesome's new all.css vs specific files? Users who want to optimise performance still have the opportunity to remove all unused CSS with Hugo's PostCSS/PurgeCSS feature.

For open source projects, it looks like FA recommend including these 4 CSS files: https://fontawesome.com/docs/web/setup/host-yourself/webfonts#version-5-compatibility . If we go that route, it might be a good idea to use Hugo's CSS processing capability to concatenate those 4 CSS files together... Also, anyone wishing to use the Pro icons should still be able to do so by adding the Pro-specific CSS via the Custom Head feature.

[rodrigoalcarazdelaosa]

For my part then, I'd include now the whole all.min.css file, which gives us compatibility with v5 too, since we have PostCSS and Font Awesome may become optional once Bootstrap 5 is added. Let me know @gcushen if anything else is needed to merge this PR. Thanks!