Bump stylelint-config-standard from 20.0.0 to 21.0.0 in /client

Bumps [stylelint-config-standard](https://github.com/stylelint/stylelint-config-standard) from 20.0.0 to 21.0.0.
- [Release notes](https://github.com/stylelint/stylelint-config-standard/releases)
- [Changelog](https://github.com/stylelint/stylelint-config-standard/blob/master/CHANGELOG.md)
- [Commits](stylelint/stylelint-config-standard@20.0.0...21.0.0)

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