[BUG] [csharp] Obsolete build warnings after PR to update to RestSharp 112.0.0 #19525

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Description

Due to vulnerabilities in the current generated RestSharp version

• [csharp] Update restsharp to the latest stable version #19387

I tried to update 7.8.0 generated C# client to RestSharp 112.0.0 as shown in

• Update C# restsharp to 112.0.0 #19507

But this produced two build warnings. For projects that treat warnings as errors in CI, this will fail the build.

ApiClient.cs(400,17): warning CS0618: 'RestClientOptions.MaxTimeout' is obsolete: 'Use Timeout instead.'
ApiClient.cs(472,24): warning CS0618: 'RestClientExtensions.Deserialize<T>(IRestClient, RestResponse)' is obsolete: 'Please use the async overload with a cancellation token'

The related upstream changes in RestSharp:

• Re-instate old methods, but as obsolete restsharp/RestSharp#2228
• Timeout as TimeSpan, Support custom request timeout restsharp/RestSharp#2078
• Wrapping up interceptors, fixing typos, removing Moq restsharp/RestSharp#2141

Here are where these obsolete methods are still used within openapi-generator main branch:

openapi-generator version

7.8.0 (with #19507 backport manually applied)