fix(plugin-sass): sassOptions type should defaults to modern API

[chenjiahan]

Summary

sassOptions type should defaults to modern API, because sass-loader >= 16.0.0 uses modern API by default.

Related Links

https://github.com/webpack-contrib/sass-loader/releases/tag/v16.0.0
#2708

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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