minor #13932 Fixed mention of minimum PHP version for v4.x (FlyingDR)

This PR was merged into the 4.4 branch.

Discussion
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Fixed mention of minimum PHP version for v4.x

Minimum PHP version for Symfony 4.x is [7.1.3](https://github.com/symfony/symfony/blob/4.4/composer.json#L19), but documentation doesn't reflect this change and keep same PHP version as 3.x.

Commits
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8ddf98d Fixed minimum PHP version for v4.4

Author: javiereguiluz

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
 ~~~~~~~~~~~~~