Fix testbench minimum version for prefer-lowest compatibility

Bump orchestra/testbench minimum versions to avoid buggy early releases
that cause "undeclared static property" errors in CI.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: aarondfrancis

composer.json

@@ -18,7 +18,7 @@
     "require-dev": {
         "mockery/mockery": "^1.6",
         "phpunit/phpunit": "^10.5|^11.0",
-        "orchestra/testbench": "^8.0|^9.0|^10.0"
+        "orchestra/testbench": "^8.21|^9.5|^10.0"
     },
     "autoload": {
         "psr-4": {