Bump postcss and @storybook/react (#9)

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [@storybook/react](https://github.com/storybookjs/storybook/tree/HEAD/code/renderers/react). These dependencies need to be updated together.


Updates `postcss` from 8.4.21 to 8.4.31
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.4.21...8.4.31)

Updates `@storybook/react` from 6.5.13 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]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]