Rename `concept` to `classifier` in node serialization

[enikao]

As decided on 2023-09-29, we change the name of concept to classifier in node serialization.

Thus, nodes look like this:

{
  "id": "aaa",
  "classifier": {
    "language": "myLanguage",
    "version": "2",
    "key": "myConceptId"
  },
  "properties": {},
  "children": {},
  "references": {},
  "annotations": [],
  "parent": null
}

Rationale:

• We want to keep the same serialization format for annotation instances and concept instances
• Pointing from an annotation instance to its defining annotation via concept is confusing
• In M3, the common supertype of Annotation and Concept is Classifier

[joswarmer]

Should close this