Chore(deps): Bump postcss, css-loader and resolve-url-loader in /legacy

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [css-loader](https://github.com/webpack-contrib/css-loader) and [resolve-url-loader](https://github.com/bholloway/resolve-url-loader/tree/HEAD/packages/resolve-url-loader). These dependencies need to be updated together.


Updates `postcss` from 7.0.36 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@7.0.36...8.4.31)

Updates `css-loader` from 3.6.0 to 6.8.1
- [Release notes](https://github.com/webpack-contrib/css-loader/releases)
- [Changelog](https://github.com/webpack-contrib/css-loader/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/css-loader@v3.6.0...v6.8.1)

Updates `resolve-url-loader` from 3.1.4 to 5.0.0
- [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/5.0.0/packages/resolve-url-loader)

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

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