Update PHP version is README.md (yiisoft#19375)

Author: charescape

README.md

@@ -19,7 +19,7 @@ Installation
 ------------
 
 - The minimum required PHP version of Yii is PHP 5.4.
-- It works best with PHP 7.
+- It works best with PHP 8.
 - [Follow the Definitive Guide](https://www.yiiframework.com/doc-2.0/guide-start-installation.html)
 in order to get step by step instructions.