Update guzzlehttp/psr7 requirement from ^2.0 to ^2.1
Update guzzlehttp/psr7 requirement from ^2.0 to ^2.1

Updates the requirements on [guzzlehttp/psr7](https://github.com/guzzle/psr7) to permit the latest version.
- [Release notes](https://github.com/guzzle/psr7/releases)
- [Changelog](https://github.com/guzzle/psr7/blob/master/CHANGELOG.md)
- [Commits](https://github.com/guzzle/psr7/compare/2.0.0...2.1.0)

---
updated-dependencies:
- dependency-name: guzzlehttp/psr7
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
slimphp/Slim