[BUG] [CSHARP-NETCORE] format: date is not serialised into OpenAPI full-date format, includes unexpected timestamp.

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?

Description

When generating the client from csharp-netcore template and the parameter is of type: string, format: date it should serialize into the full-date format (https://xml2rfc.tools.ietf.org/public/rfc/html/rfc3339.html#anchor14) : YYYY-MM-DD

At present the template serializes as: YYYY-MM-DDTHH:mm:ss.zzz (date with timestamp)

openapi-generator version

5.3.1 csharp-netcore template

OpenAPI declaration file content or URL

A minimal example of the section of OpenAPI that would generate this:

 - in: query
    name: start
    required: false
    schema:
      type: string
      example: "2018-09-17"
      format: date

Related issues/PRs

Similar issue in a different library https://github.com/RicoSuter/NSwag/issues/1211

Suggest a fix

• Serialize type: string, format: date into the correct API standard defined by full-date

Start by generating a client with the csharp-netcore template from the minimal OpenAPI declaration in the issue, then trace serialization for an optional query parameter with type string and format date. Look for the template's date handling and any related generator tests. Done means the generated client sends the value as YYYY-MM-DD without a timestamp.