[csharp] Address MaxTimeout obsolete build warning

[alec-petersen]

@wing328 , this adds that option to enable use of MaxTimeout over Timeout, but it defaults to the non-obsolete option by default. Fix #19525.

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[alec-petersen]

Good news @wing328! We can actually simplify down and lose the fallback config option. RestSharp already disregards MaxTimeout and uses the Timeout value, so setting Timeout will have the desired effect of using MaxTimeout.

[Obsolete("Use Timeout instead.")]
public int MaxTimeout {
    get => (int) (Timeout?.TotalMilliseconds ?? 0);
    set => Timeout = TimeSpan.FromMilliseconds(value);
}

[wing328]

tests passed via https://ci.appveyor.com/project/WilliamCheng/openapi-generator/builds/50804786

[wing328]

Added an option useIntForTimeout for fallback via https://github.com/OpenAPITools/openapi-generator/pull/20069, e.g. --addititional-properties useIntForTimeout=true in CLI