Update slim/psr7 requirement from ^1.6 to ^1.8

Updates the requirements on [slim/psr7](https://github.com/slimphp/Slim-Psr7) to permit the latest version.
- [Release notes](https://github.com/slimphp/Slim-Psr7/releases)
- [Commits](slimphp/Slim-Psr7@1.6...1.8.0)

---
updated-dependencies:
- dependency-name: slim/psr7
  dependency-version: 1.8.0
  dependency-type: direct:development
...

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

Author: dependabot[bot]

composer.json

@@ -67,7 +67,7 @@
         "phpstan/phpstan": "^1 || ^2",
         "phpunit/phpunit": "^9.6 || ^10 || ^11 || ^12",
         "slim/http": "^1.3",
-        "slim/psr7": "^1.6",
+        "slim/psr7": "^1.8",
         "squizlabs/php_codesniffer": "^3.10",
         "vimeo/psalm": "^5 || ^6"
     },