Bump the postcss group across 1 directory with 3 updates

Bumps the postcss group with 3 updates in the / directory: [cssnano](https://github.com/cssnano/cssnano), [cssnano-preset-default](https://github.com/cssnano/cssnano) and [postcss](https://github.com/postcss/postcss).


Updates `cssnano` from 7.1.7 to 8.0.1
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano@7.1.7...cssnano@8.0.1)

Updates `cssnano-preset-default` from 7.0.15 to 8.0.1
- [Release notes](https://github.com/cssnano/cssnano/releases)
- [Commits](https://github.com/cssnano/cssnano/compare/cssnano-preset-default@7.0.15...cssnano-preset-default@8.0.1)

Updates `postcss` from 8.5.14 to 8.5.15
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@8.5.14...8.5.15)

---
updated-dependencies:
- dependency-name: cssnano
  dependency-version: 8.0.1
  dependency-type: direct:development
  update-type: version-update:semver-major
  dependency-group: postcss
- dependency-name: cssnano-preset-default
  dependency-version: 8.0.1
  dependency-type: direct:development
  update-type: version-update:semver-major
  dependency-group: postcss
- dependency-name: postcss
  dependency-version: 8.5.15
  dependency-type: direct:development
  update-type: version-update:semver-patch
  dependency-group: postcss
...

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Author: dependabot[bot]