Bump next-sitemap from 2.5.4 to 2.5.20

Bumps [next-sitemap](https://github.com/iamvishnusankar/next-sitemap) from 2.5.4 to 2.5.20.
- [Release notes](https://github.com/iamvishnusankar/next-sitemap/releases)
- [Commits](iamvishnusankar/next-sitemap@v2.5.4...v2.5.20)

---
updated-dependencies:
- dependency-name: next-sitemap
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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Author: dependabot[bot]