Breaking change: remove the builtin autoprefixer ability of CSS

[hyf0]

Unreasonable designs resulting from the need to quickly support the business do not conform to the principles of the existing architecture design and should be removed.

Another purpose of this change is to align the related logic with Webpack.

Users would need to use postcss-loader and autoprefixer themselves just like how Webpack users doing now.

[chenjiahan]

Will Rspack provide migration guides for these breaking changes?

[hyf0]

Yes, we should.

[hyf0]

#3874