Fixed minimum PHP version for v4.4

Minimum PHP version for Symfony 4.4 is 7.1.3, but documentation doesn't reflect this change and keep same version as 3.x

Author: FlyingDR

contributing/code/pull_requests.rst

@@ -25,7 +25,7 @@ Before working on Symfony, setup a friendly environment with the following
 software:
 
 * Git;
-* PHP version 5.5.9 or above.
+* PHP version 7.1.3 or above.
 
 Configure Git
 ~~~~~~~~~~~~~