clarify allowed values / vocabulary for targetGroup metadata field

[stefanprobst]

currently, we allow four values for the targetGroup metadata field (which we think come from some EOSC vocabulary):

https://github.com/DARIAH-ERIC/dariah-campus/blob/main/src/cms/collections/posts.collection.ts#L175-L199

practically, all of our resources use the default value of "Domain researchers".

we should clarify which vocabulary, if any, we want to use for the targetGroup field, and ensure that the allowed values cover all usecases. for example we had two invalid values where "Lecturers" were added manually.