minor #20731 DX: Update PHP CS Fixer config file (keradus)

This PR was merged into the 2.7 branch.

Discussion
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DX: Update PHP CS Fixer config file

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | n/a
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

For a good start, updated config for php cs fixer v2.
I would guess that fabbot needs to be updated ;)

Commits
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a0e305b Update PHP CS Fixer config file

.php_cs.dist

@@ -1,15 +1,14 @@
 <?php
 
-return Symfony\CS\Config\Config::create()
-    ->setUsingLinter(false)
-    ->setUsingCache(true)
-    ->fixers(array(
-        'long_array_syntax',
-        'php_unit_construct',
-        'php_unit_dedicate_assert',
+return PhpCsFixer\Config::create()
+    ->setRules(array(
+        '@Symfony' => true,
+        '@Symfony:risky' => true,
+        'array_syntax' => array('syntax' => 'long'),
     ))
-    ->finder(
-        Symfony\CS\Finder\DefaultFinder::create()
+    ->setRiskyAllowed(true)
+    ->setFinder(
+        PhpCsFixer\Finder::create()
             ->in(__DIR__)
             ->exclude(array(
                 // directories containing files with content that is autogenerated by `var_export`, which breaks CS in output code