Use new PHP styleguide (PSR-12) over PSR-2

The original intent was to pick a styleguide, we already follow it but
this makes this explicit.

CODINGSTYLE.md

@@ -1,4 +1,4 @@
 DOMjudge project coding style guideline
 =======================================
 
-We follow the [PSR-2 coding style](https://www.php-fig.org/psr/psr-2/).
+We follow the [PSR-12 coding style](https://www.php-fig.org/psr/psr-12/).