Administration: Modernize admin color scheme SASS files.

This updates the admin color scheme `.sass` files to address some deprecated notices caused by upstream changes.

- `string` and `colors` are no longer globally available functions.
- `@import` is deprecated in favor of `@use`.

There are still a few notices that are output, however this will require an update to the `grunt-sass` package to resolve.

Props MattyRob.
Fixes #62323.

git-svn-id: https://develop.svn.wordpress.org/trunk@59656 602fd350-edb4-49c9-b593-d223f7449a82

package.json

@@ -66,7 +66,7 @@
 		"prettier": "npm:[email protected]",
 		"qunit": "~2.23.1",
 		"react-refresh": "0.14.0",
-		"sass": "1.79.6",
+		"sass": "1.83.4",
 		"sinon": "16.1.3",
 		"sinon-test": "~3.1.6",
 		"source-map-loader": "5.0.0",