[csharp] Handle query param encoding issues in .net9

[mohamuni]

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In .net9 SDK URL Encoding is done as part of HttpUtility.ParseQueryString() method removing dependency on HttpUtility.UrlEncode() method.

Present implementation double encodes the URL for query parameters resulting in wrong URL's.

In order to avoid double encoding in .net9 this PR blocks use of HttpUtility.UrlEncode() for .net9 SDK

[wing328]

cc @mandrean (2017/08) @shibayan (2020/02) @Blackclaws (2021/03) @lucamazzanti (2021/05) @iBicha (2023/07)

[wing328]

appveyor tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/51527181