Removing superfluous DateTimeHelper.FromOADate, added IASyncDispoable call to async writer, minor other fixes

[michelebastione]

• Resolves C# 有现成的System.DateTime.FromOADate方法问什么不用，是有什么特殊处理吗？ #592: The implementation of DateTimeHelper.FromOADate was equivalent to the standard DateTime.FromOADate already present in the framework. Although not being identical due to the latter using magic numbers instead of calculating some of the quantities, unwrapping those calculations showed a 1-1 match.
• Resolves Increase support to Async Xlsx write #605: Changed using statements to await using when compiling for .NET5+ so that DisposeAsync is called instead of Dispose
• Closes Exception message unclear #503 by clearing up the exception message

Edit: Changed NotImplementedException assertions in some tests to NotSupportedException after the modification in the previous commit that also exchanged the two exception types