Update phpunit to 9.6.34 to fix CVE-2024-5963 (#82)

Addresses unsafe deserialization vulnerability in PHPT code coverage
handling (GHSA-x3fh-mp37-85vr). Bumps minimum constraint to ^9.6.33
to prevent installing vulnerable versions.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: AaronAtDuo

composer.json

@@ -15,7 +15,7 @@
     "duosecurity/duo_universal_php": ">=1.1.1"
   },
   "require-dev": {
-    "phpunit/phpunit": "^9.0",
+    "phpunit/phpunit": "^9.6.33",
     "squizlabs/php_codesniffer": "3.*",
     "wp-coding-standards/wpcs": "^3.0",
     "10up/wp_mock": "^1.0",