Update squizlabs/php_codesniffer requirement from ^3.8 to ^3.9

Updates the requirements on [squizlabs/php_codesniffer](https://github.com/PHPCSStandards/PHP_CodeSniffer) to permit the latest version.
- [Release notes](https://github.com/PHPCSStandards/PHP_CodeSniffer/releases)
- [Changelog](https://github.com/PHPCSStandards/PHP_CodeSniffer/blob/master/CHANGELOG.md)
- [Commits](PHPCSStandards/PHP_CodeSniffer@3.8.0...3.9.0)

---
updated-dependencies:
- dependency-name: squizlabs/php_codesniffer
  dependency-type: direct:development
...

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

composer.json

@@ -25,7 +25,7 @@
     "require-dev": {
         "phpunit/phpunit": "^9.6",
         "slim/psr7": "^1.6",
-        "squizlabs/php_codesniffer": "^3.8"
+        "squizlabs/php_codesniffer": "^3.9"
     },
     "autoload": {
         "psr-4": {