Update Font Awesome from v4 to v6

We need to re-evaluate Font Awesome and how we will upgrade it.

The problem with Font Awesome v4 is that we're missing up-to-date social media icons (e.g. X, Mastodon.) Upgrading will give us a much larger repository of icons in our UIs.

We will also need to figure out how to provide these icons on our PHP site without redistributing. Currently, we rely on committing build artifacts to the PHP repos. This would count as redistribution.

This is not a problem with Hugo, as the icons get dumped into the unstaged build directory. They never become part of our source code.