Update MapBox dependencies

[AAverin]

iOS can go 5.6.0 => 6.1.0
https://github.com/mapbox/mapbox-gl-native-ios/tags

Android can go 9.2.0 => 9.4.0
https://github.com/mapbox/mapbox-gl-native-android/tags

Should resolve some of the performance issues on iOS emulators (at least)

[systemed]

iOS from 6.0.0, and Android from 9.3.0, use a closed source binary (see https://github.com/mapbox/mapbox-gl-native-ios/releases/tag/ios-v6.0.0, https://github.com/mapbox/mapbox-gl-native-android/blob/master/CHANGELOG.md#930---july-15-2020). It would be good to retain the option of the earlier versions, particularly for self-hosted tiles.

[AAverin]

@tobrun @m0nac0
What is the process of updaing mapbox dependencies?
Do you accept PRs for that or this part is fully managed by the team? I remember @m0nac0 saying something about that in one of the PRs.

Problem I still have is performance on iOS emulators. I can't create screenshots for the store because of that.

[AAverin]

With the recent release of MapBox v10 this becomes even more pressing for me.
Any way I can help?

[gabeschine]

Newer versions of the SDK require having an SDK download key from MapBox. It's relatively easy to make one and grab the SDK, but it might not be possible to upgrade this public package.

I think the best we might be able to do is provide instructions for people to generate their own key and configure the package so that it downloads & builds correctly. For any users that want something working right out of the box, though, this could be the last version.

I upgrade my package locally, had to make a few tweaks to the Dart code, and everything works fine.

[AAverin]

@gabeschine this could be a separate package with special instructions.
but I think the update should be made in the main repository rather then many people doing this in their own forks