Update to PostCSS 8

[jeddy3]

Which issue, if any, is this issue related to?

Closes #4942
Closes #5276

Is there anything in the PR that needs further explanation?

This is the long-lived branch to be merged in v14.

[hudochenkov]

Here's how we can temporarily silence plugin warnings in Jest:

const originalWarn = console.warn.bind(console.warn);

beforeAll(() => {
  console.warn = (message?, ...optionalParams) => {
    if (message.toString().includes('postcss.plugin was deprecated. Migration guide')) {
      return;
    }

    originalWarn(message, ...optionalParams);
  };
});

afterAll(() => {
  console.warn = originalWarn;
});

We probably could put it js-file for setupFilesAfterEnv.

[jeddy3]

Sounds good. It'll clean up the output while we work on fixing the failing tests.

Do you want to push a commit to this branch for it?

[hudochenkov]

This code doesn't work ¯\_(ツ)_/¯ Neither in setupFilesAfterEnv or in test file itself. I think it's due jest-preset and testRule. Because I used this code in regular Jest setup to suppress warnings.

https://krasimirtsonev.com/blog/article/jest-mock-console-methods this method also doesn't work.

We could temporarily use https://www.npmjs.com/package/patch-package to remove this warning from PostCSS.

[jeddy3]

I believe this is now ready for review.