[DowngradePhp74] Skip DowngradeCovariantReturnTypeRector on child return parent

[samsonasik]

@TomasVotruba this is the failing test case that probably related with removed return parent, the test case is different use case, while it currently the failing test case return:

from

    public function run(): parent
    {

to:

+    /**
+     * @return parent
+     */
+    public function run(): \Rector\Tests\DowngradePhp74\Rector\ClassMethod\DowngradeCovariantReturnTypeRector\Fixture\AParent

which while different probably different usecase, it should be skipped.