Bump @astrojs/mdx from 3.1.8 to 3.1.9

Bumps [@astrojs/mdx](https://github.com/withastro/astro/tree/HEAD/packages/integrations/mdx) from 3.1.8 to 3.1.9.
- [Release notes](https://github.com/withastro/astro/releases)
- [Changelog](https://github.com/withastro/astro/blob/main/packages/integrations/mdx/CHANGELOG.md)
- [Commits](https://github.com/withastro/astro/commits/@astrojs/mdx@3.1.9/packages/integrations/mdx)

---
updated-dependencies:
- dependency-name: "@astrojs/mdx"
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Author: dependabot[bot]