Update orchestra/testbench requirement from ^8.3 to ^8.15

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/develop/CHANGELOG-8.x.md)
- [Commits](orchestral/testbench@v8.3.0...v8.15.0)

---
updated-dependencies:
- dependency-name: orchestra/testbench
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -53,7 +53,7 @@
     "require-dev": {
         "laravel/legacy-factories": "^1.3",
         "mockery/mockery": "^1.5",
-        "orchestra/testbench": "^8.3",
+        "orchestra/testbench": "^8.15",
         "phpunit/phpunit": "^9.5.21"
     },
     "config": {