Breaking change: remove support for @rspack/postcss-loader

[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.

@rspack/postcss-loader is not used as postcss-loader. The package is to deal with unsupported css module syntax of SWC.

Currently, there is only one project who use this to deal with unsupported css module syntax of SWC. When users meet unsupported css module syntax, we want to fix it in a right way, which is contribute to SWC, rather than add hacky code.

[hyf0]

#3873