Remove outdated dependency that holds back GraphQL to an old version

[nicolaes]

Is your feature request related to a problem? Please describe. The upload library mentioned in #57 cannot be integrated, because GraphQL is at 4.6.1

Describe the solution you'd like Remove GraphQL.Server.Transports.AspNetCore.SystemTextJson dependency that holds back updating to GraphQL 7. Migrate code to use the new GraphQL version.

Describe alternatives you've considered None

Additional context GraphQL at 4.6.1 (project.assets.json):

Migration notice from GraphQL NuGet:

[nicolaes]

Already fixed by #80