Update/Fix Font Awesome icons

[stefan6419846]

While looking through the source code, I stumbled upon the ​https://github.com/django/django/tree/main/django/contrib/admin/static/admin/img directory.

From reading the provided information, these files were originally created by the Font Awesome project, although being converted to standalone SVG files by some third-party. As far as I can see, they have received some modifications for Django to set some other color for example.

There possibly are multiple license-related aspects which show up here (Disclaimer: IANAL):

• Splitting the big SVG file as done by Font-Awesome-SVG-PNG might be considered derivative work with a new name per OFL-1.1 and at least is covered by the weak copyleft effect of the OFL-1.1. The MIT LICENSE file provided in the directory is clearly wrong here.
• Changing the color of individual files or re-arranging them can more likely be considered derivative work, usually requiring using a different name under the terms of the OFL-1.1 and applying the OFL-1.1 to these files.

Given these observations, I would propose to update this directory accordingly:

1. Update Font Awesome to the latest version. Desktop releases for version 6.6.0 already provide individual SVG files which actually have a license comment inside as well.
2. Update the README and LICENSE file accordingly to indeed include the correct license text, id est the OFL-1.1 one. The README ideally documents which changes were done to which original version.