Bump sanitize-html and gatsby-transformer-remark

Bumps [sanitize-html](https://github.com/apostrophecms/sanitize-html) to 2.17.0 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 `sanitize-html` from 1.27.5 to 2.17.0
- [Changelog](https://github.com/apostrophecms/sanitize-html/blob/main/CHANGELOG.md)
- [Commits](https://github.com/apostrophecms/sanitize-html/commits/2.17.0)

Updates `gatsby-transformer-remark` from 2.8.40 to 6.15.0
- [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@6.15.0/packages/gatsby-transformer-remark)

---
updated-dependencies:
- dependency-name: sanitize-html
  dependency-version: 2.17.0
  dependency-type: indirect
- dependency-name: gatsby-transformer-remark
  dependency-version: 6.15.0
  dependency-type: direct:production
...

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Author: dependabot[bot]