Bump postcss and postcss-header (#1174)

Bumps [postcss](https://github.com/postcss/postcss) and [postcss-header](https://github.com/fengyuanchen/postcss-header). These dependencies needed to be updated together.

Updates `postcss` from 7.0.35 to 8.1.10
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/compare/7.0.35...8.1.10)

Updates `postcss-header` from 2.0.0 to 3.0.1
- [Release notes](https://github.com/fengyuanchen/postcss-header/releases)
- [Changelog](https://github.com/fengyuanchen/postcss-header/blob/master/CHANGELOG.md)
- [Commits](https://github.com/fengyuanchen/postcss-header/compare/v2.0.0...v3.0.1)

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