MAINTAINERS.yml: update maintainer and collaborators for Xen platform

During the review of Xen drivers enhancements, it was found that
@povergoing (previous maintainer) and @SgrrZhf (one of the collaborators)
are no longer active on GitHub. After discussions with other collaborators
(@lorc and @luca-fancellu) and in PR #82810, we decided to assign these
activities to me as one of the main developers for this part of Zephyr.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>