[BUG][aspnetcore] Fix ASP.NET Core server DateTimeOffset type mapping

[seniorquico]

Following the changes in #15967, this fixes the DateTimeOffset option in the ASP.NET Core server stub template. Notably, the setTypeMapping method overload no longer does anything that the base method doesn't already set up. But the existing overload overrides and breaks the DateTimeOffset mapping, forcing it always to DateTime. I compiled locally and ran the generator, confirming that this indeed produces a DateTimeOffset in a generated server stub.

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