[csharp] Address obsolete build warnings in ApiClient

[alec-petersen]

See issue #19525

• Non-async overload of Deserialize is obsolete, RestSharp suggests using async overload of Deserialize instead.

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[alec-petersen]

Tagging @mandrean @shibayan @Blackclaws @lucamazzanti @iBicha

[wing328]

appveyor tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/50782894

[alec-petersen]

@wing328 Undid the timeout change, moved it to #19854

[wing328]

thanks

tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/50788981