[TASK] Improve readability of php-cs-fixer configuration (#1553)

We now use `addRules()` method instead of fiddling around with an
array of rules.
It allows us to overwrite existing rules, or even disable existing rules
by setting them to `false`.

.php-cs-fixer.php

@@ -5,11 +5,11 @@
 
 $config = CsFixerConfig::create();
 
-// This is required as long as we are on PHPUnit 9.x. It can be removed after the switch to PHPUnit 10.x.
-// @see https://github.com/PHP-CS-Fixer/PHP-CS-Fixer/issues/8337
-$rules = $config->getRules();
-$rules['php_unit_test_case_static_method_calls'] = ['call_type' => 'self', 'methods' => ['createStub' => 'this']];
-$config->setRules($rules);
+$config->addRules([
+    // This is required as long as we are on PHPUnit 9.x. It can be removed after the switch to PHPUnit 10.x.
+    // @see https://github.com/PHP-CS-Fixer/PHP-CS-Fixer/issues/8337
+    'php_unit_test_case_static_method_calls' => ['call_type' => 'self', 'methods' => ['createStub' => 'this']],
+]);
 
 // @TODO 4.0 no need to call this manually
 $config->setParallelConfig(ParallelConfigFactory::detect());