[BUG] [csharp-functions] Generator does not apply useNewtonsoft parameter

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Running csharp-functions server generator with useNewtonsoft=false generates code using Newtonsoft. This is working with aspnetcore server generator but does not apply csharp-functions generator.

openapi-generator version

7.14.0

OpenAPI declaration file content or url

can be reproduced with the petstore file

Generation Details

openapi-generator-cli generate -i <file path> -g csharp-functions --additional-properties=useNewtonsoft=false

Steps to reproduce

run the command above

Suggest a fix

Expecting generated code be using System.Text.Json

Start by reproducing the issue with the csharp-functions generator and the provided command using the petstore file. Inspect how the generator handles the useNewtonsoft additional property; done means useNewtonsoft=false produces code using System.Text.Json instead of Newtonsoft.