Sass: fix deprecated @import and global function usage

[wunc]

See https://sass-lang.com/documentation/breaking-changes/import/

Deprecation Warning [import]: Sass @import rules are deprecated and will be removed in Dart Sass 3.0.0.

More info and automated migrator: https://sass-lang.com/d/import

  ╷
5 │ @import 'variables';
  │         ^^^^^^^^^^^
  ╵
    stdin 5:9  root stylesheet

and

Deprecation Warning [global-builtin]: Global built-in functions are deprecated and will be removed in Dart Sass 3.0.0.
Use color.adjust instead.

More info and automated migrator: https://sass-lang.com/d/import

   ╷
61 │ $utd-warm-gray1: lighten($utd-warm-gray2, 10%);
   │                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   ╵
    resources/assets/sass/_variables.scss 61:18  @import
    stdin 5:9                                    root stylesheet