Bump postcss and resolve-url-loader

Bumps [postcss](https://github.com/postcss/postcss) and [resolve-url-loader](https://github.com/bholloway/resolve-url-loader/tree/HEAD/packages/resolve-url-loader). These dependencies needed to be updated together.

Updates `postcss` from 7.0.35 to 8.4.33
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.35...8.4.33)

Updates `postcss` from 8.2.7 to 8.4.33
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.35...8.4.33)

Updates `resolve-url-loader` from 3.1.2 to 4.0.0
- [Release notes](https://github.com/bholloway/resolve-url-loader/releases)
- [Changelog](https://github.com/bholloway/resolve-url-loader/blob/v5/packages/resolve-url-loader/CHANGELOG.md)
- [Commits](https://github.com/bholloway/resolve-url-loader/commits/4.0.0/packages/resolve-url-loader)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: resolve-url-loader
  dependency-type: indirect
...

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