[Discussion] Remove Dart (1)

Created by: agilob

Openapi supports dart and dart2. Language didn't have many breaking changes between versions these and it can only add confusion, never seen anyone referring to Dart as dart2, it's always dart or dartlang.

dart generator doesn't seem to be tested properly and it shares generator code with dart2
there is no dart1 or dart2 specific schemas: dart-jaguar-openapi-flutter_proto.yaml dart-jaguar-openapi_proto.yaml dart-openapi-browser-client.yaml dart-openapi.yaml dart-jaguar-openapi-flutter.yaml dart-jaguar-openapi.yaml dart-openapi-flutter.yaml dart-petstore_client_lib.yaml
dart(1) is using outdated syntax and is doing too much
dart (1) doesnt have own specific documentation - dart.md describes dart2 generator
some of the schemas, specifically say not to use dart2 generator, but dart which isn't supported

cat bin/configs/dart-openapi-flutter.yaml
generatorName: dart
outputDir: samples/client/petstore/dart/flutter_petstore/openapi
inputSpec: modules/openapi-generator/src/test/resources/3_0/petstore.yaml
templateDir: modules/openapi-generator/src/main/resources/dart
additionalProperties:
  supportDart2: "false"
  hideGenerationTimestamp: "true"
  browserClient: "false"

dart(1) is also trying to solve flutter (ui framework) generation adding extra external dependencies to generated code (also badly out of date, at least 3 years, language and framework changed a lot)
- I personally see no need to add complexity of generating flutter-specific code, openapi should generate models what can be imported by flutter

Calling active dart contributors: @noordawod @sbu-WBT @josh-burton @syky27