consider removing builtins.postcss support

[hyf0]

builtins.postcss is a workaround for not supporting postcss-loader. Now postcss-loader is supported, we could remove this feature.

[hardfist]

please add the impacts and migration way for users?

[hyf0]

impacts: users who use this function.

migration way: switch to postcss-loader.

[zoolsher]

can we print a warning and also translate the configuration to module.rules?

[hyf0]

The warning is added. I'm not sure about the printing config part. We could add the migration config in the release note.

[hyf0]

Done #3849