[DOTNET] code uses "o" for ISO8601 date formats, which is not quite right

Description

"O"/"o" in .net is a shortcut for yyyy-MM-ddThh:mm:ss.fffffffZ

but ISO8601 only specifies as precise as yyyy-MM-ddThh:mm:ss.fffZ

I expect in 99.9% of APIs that doesn't matter, but NatWest's Open Banking interface rejects dates with 7 DP of precision for the second as not an ISO 8601 Date

Swagger-codegen version

problem is in master

Swagger declaration file content or url

Command line used for generation

Steps to reproduce

Use any API with a date in it and look at the date rendered into the json

Related issues/PRs

searched issue log for ISO8601 and found nothing

Suggest a fix/enhancement

..change "o" here.. to "yyyy-MM-ddThh:mm:ss.fffZ"

modules/swagger-codegen/src/main/resources/csharp/Configuration.mustache:        public const string ISO8601_DATETIME_FORMAT = "o";
modules/swagger-codegen/src/main/resources/csharp-dotnet2/Configuration.mustache:        private const string ISO8601_DATETIME_FORMAT = "o";

and where the serializer is used, you need to specify the format to use...

modules/swagger-codegen/src/main/resources/csharp/ApiClient.mustache:

        private JsonSerializerSettings serializerSettings = new JsonSerializerSettings
        {
            ConstructorHandling = ConstructorHandling.AllowNonPublicDefaultConstructor,
            DateFormatString = "yyyy-MM-ddThh:mm:ss.fffZ"
        };

..and fix issue #10461 by including the serializerSettings in teh call to Setialise

        public String Serialize(object obj)
        {
            try
            {
                return obj != null ? JsonConvert.SerializeObject(obj, serializerSettings) : null;
            }
            catch (Exception e)
            {
                throw new ApiException(500, e.Message);
            }
        }

I've not tested this, other than by "hacking" the generated code to prove that's the issue.

Start with the C# templates in modules/swagger-codegen/src/main/resources/csharp/Configuration.mustache, csharp-dotnet2/Configuration.mustache, and csharp/ApiClient.mustache. Reproduce generation with an API containing a date, then inspect the generated JSON serialization. Done means generated clients use the requested ISO 8601 precision and pass the reported serialization path without breaking existing behavior.