[Dart] Removal of deprecated Dart generators and generated code

[agilob]

Closes #7561

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[jimschubert]

I'm all for this as it's not really a blocker (people who want to use deprecated generators can use up to 5.0.0-beta2).

cc @OpenAPITools/generator-core-team do you agree?

[agilob]

Ok then, calling maintainers
@ircecho (2017/07) @swipesight (2018/09) @jaumard (2018/09) @athornz (2019/12) @amondnet (2019/12) @noordawod @sbu-WBT @josh-burton @syky27

[wing328]

About Dart (1.x) petstore tests (deleted as part of this PR, which is fine), can we reuse/copy/plot the following to Dart2 petstore tests?

• samples/client/petstore/dart/petstore/test/tests.dart
• samples/client/petstore/dart/petstore/test/tests.html

(we can do it later with a separate PR)

[agilob]

I've checked the other tests and looks like most of it is already covered in the new (dart2) test scenarios. I can add some tests after #7585 gets int

[agilob]

The next step will be migration of dart openapi schema from 2_0 to 3_0. Just checked it, it will change a lot

[wing328]

LGTM