[csharp] do not use deprecated HttpRequestMessage.Properties (#20737) #970

Job	Run time
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/ConditionalSerialization/)	2m 8s
Build .Net projects (samples/client/petstore/csharp/generichost/standard2.0/Petstore/)	2m 24s
Build .Net projects (samples/client/petstore/csharp/restsharp/standard2.0/Petstore/)	2m 46s
Build .Net projects (samples/client/petstore/csharp/httpclient/standard2.0/Petstore/)	2m 56s
	10m 14s

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