Update slim/psr7 requirement from ^1.5 to ^1.6

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@1.5...1.6)

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

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

composer.json

@@ -67,7 +67,7 @@
         "phpstan/phpstan": "^1.9",
         "phpunit/phpunit": "^9.5",
         "slim/http": "^1.2",
-        "slim/psr7": "^1.5",
+        "slim/psr7": "^1.6",
         "squizlabs/php_codesniffer": "^3.7"
     },
     "autoload": {