Bump astro and @astrojs/starlight

Bumps [astro](https://github.com/withastro/astro/tree/HEAD/packages/astro) and [@astrojs/starlight](https://github.com/withastro/starlight/tree/HEAD/packages/starlight). These dependencies needed to be updated together.

Updates `astro` from 5.18.1 to 6.1.1
- [Release notes](https://github.com/withastro/astro/releases)
- [Changelog](https://github.com/withastro/astro/blob/main/packages/astro/CHANGELOG.md)
- [Commits](https://github.com/withastro/astro/commits/astro@6.1.1/packages/astro)

Updates `@astrojs/starlight` from 0.37.7 to 0.38.2
- [Release notes](https://github.com/withastro/starlight/releases)
- [Changelog](https://github.com/withastro/starlight/blob/main/packages/starlight/CHANGELOG.md)
- [Commits](https://github.com/withastro/starlight/commits/@astrojs/starlight@0.38.2/packages/starlight)

---
updated-dependencies:
- dependency-name: astro
  dependency-version: 6.1.1
  dependency-type: direct:development
  update-type: version-update:semver-major
- dependency-name: "@astrojs/starlight"
  dependency-version: 0.38.2
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]