[ci] Silence SASS warnings (#225061)

## Summary
Silences SASS module usage warnings around the deprecated `@import`
statements.

While the sass/scss owner teams are migrating these usages, the dev
server is clogged up with deprecation warnings. It's probably best to
disable it, because it's annoying to everyone and not necessarily to the
owners.

packages/kbn-optimizer/src/worker/webpack.config.ts

@@ -224,6 +224,7 @@ export function getWebpackConfig(
                       includePaths: [Path.resolve(worker.repoRoot, 'node_modules')],
                       sourceMap: true,
                       quietDeps: true,
+                      silenceDeprecations: ['import', 'legacy-js-api'],
                     },
                   },
                 },