Bump @astrojs/mdx from 3.1.5 to 3.1.9 (#3055)

Bumps [@astrojs/mdx](https://github.com/withastro/astro/tree/HEAD/packages/integrations/mdx) from 3.1.5 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/[email protected]/packages/integrations/mdx)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

landing/package.json

@@ -12,7 +12,7 @@
   },
   "dependencies": {
     "@astrojs/check": "^0.9.3",
-    "@astrojs/mdx": "^3.1.5",
+    "@astrojs/mdx": "^3.1.9",
     "@astrojs/rss": "^4.0.9",
     "@astrojs/sitemap": "^3.2.1",
     "@astrojs/tailwind": "^5.1.2",