Update icon_map to use the latest Font Awesome 6 icons

In MDL-76989 the Font Awesome library has been upgraded from 4.7 to 6.3. Although the syntax has slightly changed, the new version is backwards compatible (because shim has been included):

<!-- Version 4's syntax -->

<i class="fa fa-camera-retro"></i> 

<!-- Version 6's syntax -->

<i class="fa-solid fa-camera-retro"></i> 

This issue is to review the references in core to all the version 4's syntax to update them to the 6's syntax. Apart from that, some icons can be updated because the new version has more images.

For instance, with Font Awesome 4, the fa-lightbulb-o is used to highlight: https://fontawesome.com/v4/icon/lightbulb-o 

However, in Font Awesome, there is a specific icon for it ("fa-solid fa-highlighter"): https://fontawesome.com/icons/highlighter?s=solid&f=classic