support RF 7.0 output XML changes

Support the new format provided with RF 7.0, to be released mid-December 2023, in the Robot Framework related REST API endpoints (and eventually in place in the UI where the user can import the RF output.xml files).

Expected changes: https://github.com/robotframework/robotframework/blob/master/doc/releasenotes/rf-7.0a1.rst#changes-to-output-xml