[PHP] Upgrade php-cs-fixer to 2.12, enables PHP >= 7.2 support (#769)

* upgrade php-cs-fixer to 2.12

* ran petstore-security-test for php

* updating openapi3 php petstore example

samples/client/petstore/php/OpenAPIClient-php/.php_cs

@@ -1,18 +1,23 @@
 <?php
 
-return Symfony\CS\Config::create()
-    ->level(Symfony\CS\FixerInterface::PSR2_LEVEL)
+return PhpCsFixer\Config::create()
     ->setUsingCache(true)
-    ->fixers(
-        [
-            'ordered_use',
-            'phpdoc_order',
-            'short_array_syntax',
-            'strict',
-            'strict_param'
-        ]
-    )
-    ->finder(
-        Symfony\CS\Finder\DefaultFinder::create()
-            ->in(__DIR__)
+    ->setRules([
+        '@PSR2' => true,
+        'ordered_imports' => true,
+        'phpdoc_order' => true,
+        'array_syntax' => [ 'syntax' => 'short' ],
+        'strict_comparison' => true,
+        'strict_param' => true,
+        'no_trailing_whitespace' => false,
+        'no_trailing_whitespace_in_comment' => false,
+        'braces' => false,
+        'single_blank_line_at_eof' => false,
+        'blank_line_after_namespace' => false,
+    ])
+    ->setFinder(
+        PhpCsFixer\Finder::create()
+        ->exclude('test')
+        ->exclude('tests')
+        ->in(__DIR__)
     );