Rebuilding CSS from SCSS emits dart-sass deprecation warnings for upcoming v3.0.0

[jayaddison]

After updating the develop-branch dart-sass dependency to the latest-available v1.97.3, various deprecation warnings relating to the select2 SCSS source files are emitted:

$ npx grunt sass:dev
Running "sass:dev" (sass) task
Deprecation Warning [legacy-js-api]: The legacy JS API is deprecated and will be removed in Dart Sass 2.0.0.

More info: https://sass-lang.com/d/legacy-js-api

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

  ╷
9 │   @import "single";
  │           ^^^^^^^^
  ╵
    src/scss/core.scss 9:11  root stylesheet

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

   ╷
10 │   @import "multiple";
   │           ^^^^^^^^^^
   ╵
    src/scss/core.scss 10:11  root stylesheet

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

   ╷
13 │ @import "dropdown";
   │         ^^^^^^^^^^
   ╵
    src/scss/core.scss 13:9  root stylesheet

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

   ╷
49 │ @import "theme/default/layout";
   │         ^^^^^^^^^^^^^^^^^^^^^^
   ╵
    src/scss/core.scss 49:9  root stylesheet

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

   ╷
50 │ @import "theme/classic/layout";
   │         ^^^^^^^^^^^^^^^^^^^^^^
   ╵
    src/scss/core.scss 50:9  root stylesheet

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

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

  ╷
2 │   background-color: mix($selection-bg-top-color, $selection-bg-bottom-color);
  │                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  ╵
    src/scss/theme/classic/_single.scss 2:21  @import
    src/scss/theme/classic/layout.scss 5:11   @import
    src/scss/core.scss 50:9                   root stylesheet

Warning: 6 repetitive deprecation warnings omitted.


Done.

These are currently non-fatal (they're warnings, not errors) -- but indicate that updating the SCSS sources would make sense to enable future upgrades to v3.0.0 of dart-sass.