This is an enum in the original xsd while we don't output possible values
currently, so in the mean time make it mandatory, it's still a bit better than
relying on the original schema for that, and it makes sense from a business POV
to force specification of description level of an archive unit.

A lot of tests have to be updated, some of them because output value is now
different, others because we've to introduce a scheme+concept to handle the
validation, but they were not expecting that.

Closes #16687907