chore(deps): bump postcss, @docusaurus/core and @docusaurus/preset-classic

Bumps [postcss](https://github.com/postcss/postcss) to 8.5.15 and updates ancestor dependencies [postcss](https://github.com/postcss/postcss), [@docusaurus/core](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus) and [@docusaurus/preset-classic](https://github.com/facebook/docusaurus/tree/HEAD/packages/docusaurus-preset-classic). These dependencies need to be updated together.


Updates `postcss` from 8.3.6 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@8.3.6...8.5.15)

Updates `@docusaurus/core` from 2.0.0-beta.4 to 2.4.3
- [Release notes](https://github.com/facebook/docusaurus/releases)
- [Changelog](https://github.com/facebook/docusaurus/blob/main/CHANGELOG-v2.md)
- [Commits](https://github.com/facebook/docusaurus/commits/v2.4.3/packages/docusaurus)

Updates `@docusaurus/preset-classic` from 2.0.0-beta.4 to 2.4.3
- [Release notes](https://github.com/facebook/docusaurus/releases)
- [Changelog](https://github.com/facebook/docusaurus/blob/main/CHANGELOG-v2.md)
- [Commits](https://github.com/facebook/docusaurus/commits/v2.4.3/packages/docusaurus-preset-classic)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.15
  dependency-type: indirect
- dependency-name: "@docusaurus/core"
  dependency-version: 2.4.3
  dependency-type: direct:production
- dependency-name: "@docusaurus/preset-classic"
  dependency-version: 2.4.3
  dependency-type: direct:production
...

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Author: dependabot[bot]