Bump postcss and @storybook/react

Removes [postcss](https://github.com/postcss/postcss). It's no longer used after updating ancestor dependency [@storybook/react](https://github.com/storybookjs/storybook/tree/HEAD/code/renderers/react). These dependencies need to be updated together.


Removes `postcss`

Updates `@storybook/react` from 6.5.9 to 7.4.6
- [Release notes](https://github.com/storybookjs/storybook/releases)
- [Changelog](https://github.com/storybookjs/storybook/blob/next/CHANGELOG.md)
- [Commits](https://github.com/storybookjs/storybook/commits/v7.4.6/code/renderers/react)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: "@storybook/react"
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>