[PHP8.1] use `readonly` properties

currently readonly properties are private, annotated as `@readonly` and available via a getter.

since php8.1 native support for readonly properties exists.

# proposal
- [ ] remove the getter
- [ ] make the property public
- [x] make the property readonly - via #208

## attention

- there are issues when it comes to modifications in `__clone()` - -therefore, not fully rolled out
- there are extended works needed to make the test cases aware ...
- failed try: #207