Rename csharp-netcore to csharp (#15903)

* rename csharp-netcore to csharp

* rename file

* renmae modules/openapi-generator/src/main/resources/csharp-netcore

* update samples

* mv dir

* update samples

* rename csharp-netcore to csharp in appveyor.yml

* update doc

samples/client/petstore/csharp-functions/generatedSrc/Client/OpenAPIDateConverter.cs

@@ -0,0 +1,29 @@
+/*
+ * OpenAPI Petstore
+ *
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * Generated by: https://github.com/openapitools/openapi-generator.git
+ */
+
+using Newtonsoft.Json.Converters;
+
+namespace Org.OpenAPITools.Client
+{
+    /// <summary>
+    /// Formatter for 'date' openapi formats ss defined by full-date - RFC3339
+    /// see https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#data-types
+    /// </summary>
+    public class OpenAPIDateConverter : IsoDateTimeConverter
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="OpenAPIDateConverter" /> class.
+        /// </summary>
+        public OpenAPIDateConverter()
+        {
+            // full-date   = date-fullyear "-" date-month "-" date-mday
+            DateTimeFormat = "yyyy-MM-dd";
+        }
+    }
+}