[SYCL] Move proposed sycl_ext_oneapi_group_sort extension to experimental (#14531)

* Align [proposed sycl_ext_oneapi_group_sort
extension](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/proposed/sycl_ext_oneapi_group_sort.asciidoc)
extension with the implementation.
* Move proposed extension to experimental directory because it was
implemented. 
* Remove the first version of API and update feature macro.

Author: againull