Many users report the same problem with PostCSS 7 → 8 migration.

1. They updated postcss-loader
2. They ignored postcss peer dependency warning
3. npm takes old postcss 7 from some deep dependency
4. As a result they have an error about PostCSS incompatibility

Of course, we can blame users. But this is a popular error.

In postcss-cli we check postcss().version and show Add postcss to package.json error message. Should we do something like this in postcss-loader?