Silence Sass warnings from dependencies

For example we can’t fix “Using / for division outside of calc() is deprecated” in:

```
govuk_frontend_toolkit
govuk-elements-sass
```

See https://frontend.design-system.service.gov.uk/importing-css-assets-and-javascript/#silence-deprecation-warnings-from-dependencies-in-dart-sass

tasks/compile-stylesheets.mjs

@@ -66,6 +66,9 @@ export async function compileStylesheet ([modulePath, { srcPath, destPath }]) {
       file: moduleSrcPath,
       outFile: moduleDestPath,
 
+      // Turn off dependency warnings
+      quietDeps: true,
+
       // Enable source maps
       sourceMap: true,
       sourceMapContents: true,