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

lib/jest-helpers.js

@@ -142,6 +142,7 @@ async function getExamples (componentName) {
 function renderSass (options) {
   return sassRender({
     includePaths: sassPaths,
+    quietDeps: true,
     ...options
   })
 }

tasks/gulp/compile-assets.mjs

@@ -105,7 +105,7 @@ compileStylesheets.displayName = 'compile:scss'
 function compileStylesheet (stream, options = {}) {
   return stream
     .pipe(plumber(errorHandler(stream, 'compile:scss')))
-    .pipe(sass(options))
+    .pipe(sass({ ...options, quietDeps: true }))
     .pipe(postcss())
     .pipe(plumber.stop())
 }