Do not use PHPUnit 10

Seems that there might be issues with it, causing it to report that a never-returning function returns.

> PHP Fatal error:  A never-returning function must not return in /home/runner/work/http-client/http-client/vendor/phpunit/phpunit/src/Framework/Constraint/Constraint.php on line 91

https://github.com/wp-oop/http-client/actions/runs/5901854051/job/16008901130?pr=1

https://github.com/sebastianbergmann/phpunit/blob/main/src/Framework/Constraint/Constraint.php#L91

Author: XedinUnknown

composer.json

@@ -17,7 +17,7 @@
     "psr/http-factory-implementation": "*"
   },
   "require-dev": {
-    "phpunit/phpunit": "^9.0@stable | ^10.0@stable",
+    "phpunit/phpunit": "^9.0@stable",
     "vimeo/psalm": "^5.0",
     "webmozart/path-util": "^2.3@stable",
     "slevomat/coding-standard": "^6.0",