Consider (not) deprecating relational owned JSON support

[roji]

With complex types fully supported in EF 10, we'll consider deprecating owned entity support for JSON - at least until we allow persisting key properties within JSON (see #28594).

For anyone coming to this issue, we're looking for feedback on the above plan; we're specifically interested in any reasons you have for preferring owned entity JSON support over complex type JSON support. We're interested in feature gaps (e.g. X works with owned entity types but not with complex types), but even more in fundamental reasons to prefer the former (as feature gaps can be fixed).

[roji]

An [Obsolete] attribute has been added to the relevant APIs; reopening this issue to track feedback and make a final decision on whether we keep the obsoletion for 11.0.