Update slim/psr7 requirement from ^1.0 to ^1.1

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@1.0.0...1.1.0)

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

composer.json

@@ -65,7 +65,7 @@
         "phpstan/phpstan": "^0.11.5",
         "phpunit/phpunit": "^8.5",
         "slim/http": "^1.0",
-        "slim/psr7": "^1.0",
+        "slim/psr7": "^1.1",
         "squizlabs/php_codesniffer": "^3.5"
     },
     "autoload": {