[csharp] RemoteCertificateValidationCallback was not called on async methods; resolves #14608

[torgilf]

added missing line in config mapping, closes #14608

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[wing328]

samples updated via 2fe08cc

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/48388747