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](guzzle/psr7@2.0.0...2.1.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

composer.json

@@ -56,7 +56,7 @@
     "require-dev": {
         "ext-simplexml": "*",
         "adriansuter/php-autoload-override": "^1.2",
-        "guzzlehttp/psr7": "^2.0",
+        "guzzlehttp/psr7": "^2.1",
         "laminas/laminas-diactoros": "^2.8",
         "nyholm/psr7": "^1.4",
         "nyholm/psr7-server": "^1.0",