6.0/2923 Make serialization use new serializers

[ewoutkramer]

Fixes #2923.

⚠️ This PR cleans up the serialization API, mostly getting rid of similar but different serializer settings. This is probably one of the biggest batches of changes to the public surface, so it has its own section here: https://github.com/FirelyTeam/firely-net-sdk/wiki/Breaking-changes-in-6.0#breaking-changes-to-the-serializers

I have not yet done some renames because it would make reviewing harder. So after review, this still needs to be done:

• Rename CommonFhirXml/JsonSerializer -> BaseFhirXml/JsonSerializer
• CommonFhirXml/JsonSerializerExtensions -> Xml/JsonSerializerExtensions
• Of course, rename the files.

[ewoutkramer]

I have now documented all serialization functions and extensions, as requested verbally by @mmsmits.