[php] Add PSR-12 rule set (#12302)

* Upgrade PHPCS-Fixer to 3.5 version

This version requires PHP 7.4 or 8.0 which perfectly meets our supported
PHP right now.

* Rename config file to .php-cs-fixer.dist.php

This way it can be overwritten with local file(same name without "dist"
part). Also it's recommended filename from the package doc.

* Add PSR12 rule set to config

I've changed config file a bit to look like example from the doc.
Ref: https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/HEAD/doc/config.rst

* Ignore new cache format

* Remove blank line after class opening

* Set constants in models public

* Refresh samples

samples/client/petstore/php/OpenAPIClient-php/lib/ApiException.php

@@ -39,7 +39,6 @@ use \Exception;
  */
 class ApiException extends Exception
 {
-
     /**
      * The HTTP body of the server response either as Json or string.
      *