server: `SubmodelDescriptor` serialization names `supplementalSemanticIds` wrong

[hpoeche]

Issue

Currently, the serialization of SubmodelDescriptor objects, defined in server/app/adapter/jsonization.py, uses the attribute name supplementalSemanticIds (with trailing s).
This is not compliant to the schema of the API objects in v3.1.1, which is the version that the README.md lists for the API.
This version still specifies the attribute name supplementalSemanticId without a trailing "s" (this was changed in v3.1.2 of the spec)

##Tests
After solving this issue, the test commented section in the test TestRegistryObjectSerialization.test_submodel_descriptor_serialization in server/test/adapter/test_jsonization.py (introduced with #627) should be enabled to test for correct serialization