Use DateOnly in C# generator for date type

[ColinEberhardt]

In ScottLogic/openapi-forge#185, it is established that swagger distinguishes date and date-time types. The necessary changes to address this has been made in the main repo (see ScottLogic/openapi-forge#187).

To reflect these changes, C# tests are also altered in #50. However, these changes are applied on a surface level where both date and date-time strings are still represented by DateTime type in C#. In the mentioned PR, the tests conditionally convert the testing property into DateOnly on runtime in an ad-hoc way.

For context, C# introduced DateOnly type [1] in dotnet6. However, they haven't introduced the serialisation/deserialisation for this type until dotnet7 [2]. Since the test project is on dotnet6 (which is a LTS release), it is not possible to use the DateOnly type on generated code.

To address the mentioned problem, one option is to write custom ser/deser methods which can be undesirable for generated code structures. A more viable second option to bring the project to dotnet >=7 at a later stage to eliminate this limitation. The testing of date/datetime can be streamlined from that point onwards.