[BUG][C#][csharp-netcore] x-csharp-value-type doesn't help to generate nullable type

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

I'm using 'x-csharp-value-type: true' to generate nullable DateTime but this doesn't works

openapi-generator version

docker image: openapitools/openapi-generator-cli:latest

OpenAPI declaration file content or url

https://gist.github.com/dstepanov25/91fc075149b832edb8c6131a70b527fc

Generation Details

1. docker image pull openapitools/openapi-generator-cli:latest
2. docker-compose up for this file https://gist.github.com/dstepanov25/8715114c57d9a7517f9a7647e231dc5e
   As a result I'm receiving a project with such model: https://gist.github.com/dstepanov25/2194bad7e1d2ac6e06db5de060ee1466
   I'm expecting to see public DateTime? Timestamp { get; set; } to avoid msbuild warnings

Steps to reproduce

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/3693

Suggest a fix

Start with the OpenAPI declaration gist, the docker-compose generation steps, and the generated C# model gist to reproduce the nullable DateTime problem. Trace how csharp-netcore handles x-csharp-value-type, then verify that the generated model uses public DateTime? Timestamp { get; set; } without the reported MSBuild warnings.