Removing legacy serialization from log4net

[FreeAndNil]

Because of https://aka.ms/binaryformatter we consider removing ISerializable and [Serializable] from all 17 types (which currently support them).
Version 3.0.0 removed support for .netstandard (but keep it for .net462).
In a later version we want to drop the support completely and maybe switch do DataContractSerializer.

See #174 for step one.

If you use ISerializable or [Serializable] on one or more of the 17 log4net types, please leave a comment describing you use case and naming your target framework.
Please also note, whether you use BinaryFormatter.