Mark unsupported serialization implementations as obsolete to remove them in another major version

[StefanOssendorf]

As discussed with rocky we should mark

• CslaLegacyBinaryReader
• CslaLegacyBinaryWriter
  as obsolete with a hint that they will be removed in the next major version.

Furthermore we still have some DataContractSerializer (DCS) remnants in the project.
That means the two classes

• CslaXmlBinaryReader
• CslaXmlBinaryWriter
• CslaXmlWriter
• CslaXmlReader

should also be marked as obsolete.

In addition to that the static factory method creating the DCS must also be marked as obsolete.

• CslaReaderWriterFactory.GetDataContractSerializer()

[StefanOssendorf]

@rockfordlhotka Please check the last 4 reader/writes I mentioned in the issue. Marking them as obsolete would mean only CslaBinaryWriter/Reader is currently a valid implementation within CSLA itself 🤔