Update phpunit/phpunit requirement from ^8.5 || ^9.3 to ^8.5.13

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.5.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/commits/8.5.13)

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

composer.json

@@ -64,7 +64,7 @@
         "nyholm/psr7-server": "^1.0.0",
         "phpspec/prophecy": "^1.12",
         "phpstan/phpstan": "^0.12.54",
-        "phpunit/phpunit": "^8.5 || ^9.3",
+        "phpunit/phpunit": "^8.5.13",
         "slim/http": "^1.2",
         "slim/psr7": "^1.3",
         "squizlabs/php_codesniffer": "^3.5"