Switch over to the new parsers

[ewoutkramer]

Although we have our new stack of parsers available since SDK 4.0, we've been very careful to not cause incompatibilities by using them ourselves in the SDK. E.g. the DirectorySource still uses the older ITypedElement-based parsers. Also, the decade-old FhirXmlParser/FhirJsonParser still use these old parsers internally. There is one exception: the FhirClient. When we redid the FhirClient for SDK 5.0, we have also made it possible to switch to the newer parsers there, but we still defaulted to the old parsers.

It is time to pull the bandage and make the switch. We should follow FhirClient's lead and do this via IFhirSerializationEngine, so users could even provide their own. But we should also make the jump and use the new parsers by default.

One of the biggest differences is the kind of errors reported and how they are reported. The old parsers generally throw (although there is a system with IExceptionSource to change that behaviour), and they use different (and uncoded) errors. One idea might be to align the errors between the parsers as well, but it's questionable whether this provides value, since that's hardly less breaking.