fix: Update phpunit dependency to allow composer to install non abandoned package versions

Resolves composer package conflicts which result in a warning like:
```
Package php-cs-fixer/diff is abandoned, you should avoid using it. No replacement was suggested.
Package phpunit/php-token-stream is abandoned, you should avoid using it. No replacement was suggested
```

Signed-off-by: Ferdinand Thiessen <rpm@fthiessen.de>

composer.json

@@ -10,7 +10,7 @@
         }
     ],
     "require-dev": {
-        "phpunit/phpunit": "^8.5",
+        "phpunit/phpunit": "^9.5",
         "nextcloud/coding-standard": "^1.0.0",
         "nextcloud/ocp": "dev-stable25"
     },