MAINT: use fontawesome css variables instead of font names

[12rambau]

Fix #1195

Everything is in the title:

instead of relying on the name of the fonts embeded in fontawesome (which forces us to change them all if we change fa version, we now rely on the css variables injected by fontawesome distribution.

font: var(--fa-font-solid);

is equivalent to:

font-family: "Font Awesome 6 Free";
font-weight: 400;
font-size: 1em; 
line-height: 1;
font-style: normal;

I didn't changed the icon content with the SASS mixin. We would loose our capacity to reuse the icon elswhere as the variables needs to be scss compiled one. To be fair unicodes never change so I would say it's ok.

[choldgraf]

The icons look good to me - and this is a nice simplification ! FYI it looks like you have the same lockfile update in this one too