Bump yaml, postcss-loader and stylelint in /core/assets

Removes [yaml](https://github.com/eemeli/yaml). It's no longer used after updating ancestor dependencies [yaml](https://github.com/eemeli/yaml), [postcss-loader](https://github.com/webpack-contrib/postcss-loader) and [stylelint](https://github.com/stylelint/stylelint). These dependencies need to be updated together.


Removes `yaml`

Updates `postcss-loader` from 4.2.0 to 7.2.4
- [Release notes](https://github.com/webpack-contrib/postcss-loader/releases)
- [Changelog](https://github.com/webpack-contrib/postcss-loader/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/postcss-loader@v4.2.0...v7.2.4)

Updates `stylelint` from 13.12.0 to 15.6.0
- [Release notes](https://github.com/stylelint/stylelint/releases)
- [Changelog](https://github.com/stylelint/stylelint/blob/main/CHANGELOG.md)
- [Commits](stylelint/stylelint@13.12.0...15.6.0)

---
updated-dependencies:
- dependency-name: yaml
  dependency-type: indirect
- dependency-name: postcss-loader
  dependency-type: direct:development
- dependency-name: stylelint
  dependency-type: direct:development
...

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