Merge topic 'remove-header-unit-fileset-type'

7b62a83c6f cxxmodules: update the experimental UUID
6ff5cdc533 cxxmodules: remove support for `CXX_MODULE_HEADER_UNITS` filesets

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Merge-request: !8510