Update default license

[kburger]

The current value used is the non-persistent url version, which no longer resolves. The purl.org variant does resolve, making it an easy fix.

• [x] default value namespace
• [x] decision on specific license (now cc-by-nc-nd 3.0)
• [x] migration impl

[MarekSuchanek]

done via https://github.com/FAIRDataTeam/FAIRDataPoint/commit/538920130f81ee78470d17b11799f5c14ab0c909