Update dependency postcss-loader to v8.1.0

This MR contains the following updates:

Package	Type	Update	Change
postcss-loader	devDependencies	minor	8.0.0 -> 8.1.0

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Release Notes

webpack-contrib/postcss-loader (postcss-loader)

v8.1.0

Compare Source

Features

• add @rspack/core as an optional peer dependency (#​679) (512e4c3)

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