[release/6.0] Fix binary serialization of DateTime with DCS and add test.

[StephenMolloy]

Fixes: #61528 in 6.0.
This exact change was already included as part of the big DCS/NetFx alignment done in 7.0. (#71752)

Customer Impact

The WCF binary XML format written by XmlDictionaryWriter.CreateBinaryWriter allows for a binary representation of DateTime objects that is shorter and faster to deserialize than the standard string presentation. NetFx used this binary format, but .Net Core has used a string representation. That leads to much larger blobs when the serialized objects contain many DateTimes and, more importantly, significantly longer deserialization times for some customers.

Testing

A test has been added to verify the use of the binary format vs string format for DateTime.

Regression

Yes? This is a regression from 4.8, though it has always been this way in .Net Core.

Risk

Low. This PR restores the NetFx way of serializing a DateTime value... which is to let XmlWriter handle it rather than injecting our own formatting. (I believe the custom formatting was only used because XmlWriter.WriteValue() did not have an overload for DateTime in .Net Core 1.0. It has since been available since 1.1.)

[carlossanlop]

@StephenMolloy please fill out the full template, add the servicing-consider label, and send email to Tactics to request approval.

[carlossanlop]

Feedback addressed, approved by Tactics, signed-off by area owner, CI is green. Ready to merge.

Edit: Almost forgot to mention that neither System.Private.DataContractSerialization nor System.Runtime.Serialization.Xml are OOB packages. They do not have <IsPackable>true in the src csproj, and they show up in the file https://github.com/dotnet/runtime/blob/release/6.0/src/libraries/NetCoreAppLibrary.props