github/gitea: Fix type of as:totalItems being xsd:integer instead of xsd:nonNegativeInteger

rdflib implicitly sets xsd:integer as the type of Literal(42); while the
ActivityStream specification requires a xsd:nonNegativeInteger here.

This was highlighted by a bugfix in rdflib 7.1.0, as previous versions
omitted serializing the "default" datatype corresponding to the Python
type of the literal value when writing in JSON-LD.