Fix nullable reference types in ApiException for old language versions

Author: EraYaN

modules/openapi-generator/src/main/resources/csharp/libraries/generichost/ApiException.mustache

@@ -32,15 +32,15 @@ namespace {{packageName}}.{{clientPackage}}
         /// <summary>
         /// The HttpResponseMessage object
         /// </summary>
-        public System.Net.Http.HttpResponseMessage? RawResponse { get; }
+        public System.Net.Http.HttpResponseMessage{{#nrt}}?{{/nrt}} RawResponse { get; }
 
         /// <summary>
         /// Construct the ApiException from parts of the response
         /// </summary>
         /// <param name="reasonPhrase"></param>
         /// <param name="statusCode"></param>
         /// <param name="rawContent"></param>
-        public ApiException(string{{nrt?}} reasonPhrase, System.Net.HttpStatusCode statusCode, string rawContent, System.Net.Http.HttpResponseMessage? rawResponse = null) : base(reasonPhrase ?? rawContent)
+        public ApiException(string{{nrt?}} reasonPhrase, System.Net.HttpStatusCode statusCode, string rawContent, System.Net.Http.HttpResponseMessage{{#nrt}}?{{/nrt}} rawResponse = null) : base(reasonPhrase ?? rawContent)
         {
             ReasonPhrase = reasonPhrase;