Add Font Awesome 6 and remove deprecated Font Awesome 4

[dalthviz]

Follow up for the petition done at #229 (comment) and #178

[arambert]

Hi,
Thank you for this awesome work!
It would be really nice indeed to be able to use FA6 (the C version of QtAwesome already implements FA6). Do you know when this would be added in this python version?

[dalthviz]

Hi there @arambert sorry for the late response and also sorry but we don't have a specific date to make Font Awesome 6 available :/ However, if you are interested (or for anyone interested), part of the changes that need to be done should be quite similar to what was done over #86 when Font Awesome 5 was added (probably using as prefix something like fa6*)