Update `astro` and `@astrojs/mdx`

[ArakTaiRoth]

Info

We have a PR to update astro from 6.4.8 to 7.1.3 and @astrojs/mdx from 5.0.6 to 7.0.3. This is being done to resolve multiple security alerts include a high severity issue.

The PR is experiencing many issues, likely because it's a major version upgrade.

This will need actual time to upgrade and fix all related issues.

Acceptance Criteria:

1. astro is updated to 7.1.3
2. @astrojs/mdx is updated to 7.0.3
3. All issues are resolved with the new major versions