Replace asset-url with url in Sass files (#3698)

Both Sprockets and Propshaft will replace `url` with a digested
version [1]. This change allows to use these Sass files with other
Sass implementations like, Dartsass [2].

[1] rails/dartsass-rails#18 (comment)
[2] https://github.com/rails/dartsass-rails

Co-authored-by: Jacob Carlborg <jacob.carlborg@apoex.se>

Author: jacob-carlborg-apoex

vendor/assets/stylesheets/rails_admin/font-awesome.scss

@@ -12,7 +12,7 @@
   font-style: normal;
   font-weight: 900;
   font-display: block;
-  src: asset-url("rails_admin/fa-solid-900.woff2") format("woff2"), asset-url("rails_admin/fa-solid-900.ttf") format("truetype"); }
+  src: url("rails_admin/fa-solid-900.woff2") format("woff2"), url("rails_admin/fa-solid-900.ttf") format("truetype"); }
 
 .fa,
 .fas,