chore(deps): bump postcss and gatsby-transformer-remark

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.31 and updates ancestor dependency [gatsby-transformer-remark](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-transformer-remark). These dependencies need to be updated together.


Updates `postcss` from 8.4.18 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@8.4.18...8.4.31)

Updates `gatsby-transformer-remark` from 5.24.0 to 6.12.0
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-remark/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-transformer-remark@6.12.0/packages/gatsby-transformer-remark)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: gatsby-transformer-remark
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>