composer(deps-dev): update orchestra/testbench requirement

Updates the requirements on [orchestra/testbench](https://github.com/orchestral/testbench) to permit the latest version.
- [Release notes](https://github.com/orchestral/testbench/releases)
- [Changelog](https://github.com/orchestral/testbench/blob/6.x/CHANGELOG-6.x.md)
- [Commits](orchestral/testbench@v6.17.1...v6.18.0)

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

Author: dependabot[bot]

composer.json

@@ -25,7 +25,7 @@
         "statamic/cms": "^3.1"
     },
     "require-dev": {
-        "orchestra/testbench": "^6.17.1",
+        "orchestra/testbench": "^6.18.0",
         "phpunit/phpunit": "^9.5"
     },
     "config": {