`RegexTextExtractor` - remove `_backward_compatible` decorator

[anakin87]

in #10304, we removed the return_empty_on_no_match init parameter from RegexTextExtractor.

To avoid breaking the existing component, a _backward_compatible decorator that skips this init parameter was added. Users have been informed that this compatibility layer will be removed in 2.23.0.

So, after 2.22.0 is released, we should remove the _backward_compatible decorator.
We should also understand if we still want to provide some compatibility layer for deserialization (from_dict?): changing saved YAML is harder and errors are less clear.

To be handled internally.