Fixathon: Web app dependency updates part 2 (Sass) (#29037)

* Update sass-loader@16.0.2

* Update sass part 1 (actually update it)

* Update scss files to remove at-charset and use at-use instead of at-import

at-charset hasn't been needed since we switched to dart-sass, and
at-import is now deprecated. Most of this was done using sass-migrator.

We still use at-import for files imported from node_modules, but sass
doesn't seem to complain about that

* Inline at-extend rule that was causing an error

* Remove or inline at-use rules in components package

webapp/package.json

@@ -46,8 +46,8 @@
     "eslint-plugin-react-hooks": "4.6.0",
     "mini-css-extract-plugin": "2.7.5",
     "patch-package": "8.0.0",
-    "sass": "1.71.1",
-    "sass-loader": "14.1.1",
+    "sass": "1.80.5",
+    "sass-loader": "16.0.2",
     "strip-ansi": "7.1.0",
     "style-loader": "4.0.0",
     "webpack": "5.95.0",