Deprecate the Evas GL renderer and ElmFlutterView

[swift-kim]

As a continuation of #481, we will eventually remove the Evas GL rendering backend support from the Tizen embedder, to reduce our maintenance burden and focus only on EGL renderer development. The Evas GL renderer is currently used in the following cases:

• When running on watch devices (because apps that use the EGL renderer cannot be uploaded to the app store)
• The renderer type is explicitly specified in the user code (App.cs or runner.cc)
• When adding a Flutter view to an existing EFL application

This will not happen anytime soon, since Flutter apps designed for Galaxy Watch devices will no longer be able to be published on the app store after this change. If Samsung discontinues support for submitting watch apps to their app store, we may consider moving forward with this. Nothing will happen until then, and there's no action you need to take as an app developer.

[JSUYA]

Cases using the external output function should be also added to the list above.