[REQ][CSharp] Suppress build warnings from generated code

Created by: snebjorn

Is your feature request related to a problem? Please describe.

Building a C# project that references a generated http client gets pretty noisy. Here's a typical output:

warning CS0618: 'HttpRequestMessage.Properties' is obsolete: 'HttpRequestMessage.Properties has been deprecated. Use Options instead.
warning CS0618: 'HttpRequestMessage.Properties' is obsolete: 'HttpRequestMessage.Properties has been deprecated. Use Options instead.
warning CS0618: 'HttpRequestMessage.Properties' is obsolete: 'HttpRequestMessage.Properties has been deprecated. Use Options instead.
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'
warning CS8073: The result of the expression is always 'true' since a value of type 'Guid' is never equal to 'null' of type 'Guid?'

It's generated code. I don't care :)

Describe the solution you'd like

The solution is pretty simple. Suppress all warnings. Add the following project settings to https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/csharp-netcore/netcore_project.mustache

<PropertyGroup>
  ...other properties omitted
  <WarningLevel>0</WarningLevel>
  <AnalysisLevel>none</AnalysisLevel>
</PropertyGroup>

Describe alternatives you've considered

Suppressing warnings could be a flag in the generator. But I don't see why anyone would care about warnings from generated code.