Bump @astrojs/mdx from 3.1.7 to 3.1.8

Bumps [@astrojs/mdx](https://github.com/withastro/astro/tree/HEAD/packages/integrations/mdx) from 3.1.7 to 3.1.8.
- [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/[email protected]/packages/integrations/mdx)

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

Signed-off-by: dependabot[bot] <[email protected]>