Followup: <project><availability> should be its own enum relevant to projects.

[jensstalder]

<xs:element name="availability" minOccurs="1" maxOccurs="1"> For projects are "extending" the availability from properties. Which in my opinion does not make sense, since a project can't be "reserved" or "in-acquisition" or "taken". So we should change that enum to just have "active" for the moment. And we can add more in the future such as "private" if they make sense for projects.

Originally posted by @jensstalder in https://github.com/qualipool/swissrets/issues/176#issuecomment-823298932