v6.0.0 - PHP8.3 support and first class callables support

• new major release for PHP8.3
• added support for first-class callables in listeners definition:

Old definition (also supported)

    public function getSubscribedEvents(): array
    {
        return [
            'SomeClass::onSomething' => 'doSomething',
        ];
    }

    public function doSomething($param1, $param2): void {
        // do some stuff
    }

New definition:

    public function getSubscribedEvents(): array
    {
        return [
            'SomeClass::onSomething' => $this->doSomething(...),
        ];
    }

    private function doSomething($param1, $param2): void {
        // do some stuff
    }

Full Changelog: v5.0.3...v6.0.0