Update guzzlehttp/psr7 requirement from ^1.7 to ^1.8

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/1.8.2/CHANGELOG.md)
- [Commits](guzzle/psr7@1.7.0...1.8.2)

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": "^1.7",
+        "guzzlehttp/psr7": "^1.8",
         "http-interop/http-factory-guzzle": "^1.0",
         "laminas/laminas-diactoros": "^2.4",
         "nyholm/psr7": "^1.4",