docs: change the version of PHPUnit to major version only

Co-authored-by: MGatner <[email protected]>

Author: kenjis

tests/README.md

@@ -8,7 +8,7 @@ use to test your application. Those details can be found in the documentation.
 ## Requirements
 
 It is recommended to use the latest version of PHPUnit. At the time of this 
-writing we are running version 9.5.10. Support for this has been built into the 
+writing we are running version 9.x. Support for this has been built into the 
 **composer.json** file that ships with CodeIgniter and can easily be installed 
 via [Composer](https://getcomposer.org/) if you don't already have it installed globally.