Bump the next group across 1 directory with 2 updates

Bumps the next group with 2 updates in the / directory: [@next/mdx](https://github.com/vercel/next.js/tree/HEAD/packages/next-mdx) and [next](https://github.com/vercel/next.js).


Updates `@next/mdx` from 14.2.5 to 15.1.0
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](https://github.com/vercel/next.js/commits/v15.1.0/packages/next-mdx)

Updates `next` from 14.2.13 to 15.1.0
- [Release notes](https://github.com/vercel/next.js/releases)
- [Changelog](https://github.com/vercel/next.js/blob/canary/release.js)
- [Commits](vercel/next.js@v14.2.13...v15.1.0)

---
updated-dependencies:
- dependency-name: "@next/mdx"
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: next
- dependency-name: next
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: next
...

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Author: dependabot[bot]