Update nyholm/psr7 requirement from ^1.7 to ^1.8

Updates the requirements on [nyholm/psr7](https://github.com/Nyholm/psr7) to permit the latest version.
- [Release notes](https://github.com/Nyholm/psr7/releases)
- [Changelog](https://github.com/Nyholm/psr7/blob/master/CHANGELOG.md)
- [Commits](Nyholm/psr7@1.7.0...1.8.0)

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

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

composer.json

@@ -60,7 +60,7 @@
         "httpsoft/http-message": "^1.0",
         "httpsoft/http-server-request": "^1.0",
         "laminas/laminas-diactoros": "^2.17",
-        "nyholm/psr7": "^1.7",
+        "nyholm/psr7": "^1.8",
         "nyholm/psr7-server": "^1.0",
         "phpspec/prophecy": "^1.17",
         "phpspec/prophecy-phpunit": "^2.0",