👷Fix the codespace by upgrading the docker image

[ouvreboite]

Currently, starting the example in the Codespace does not work:

# cd example/
# flutter run -d web-server --web-hostname=0.0.0.0
Downloading Web SDK...                                           2,735ms
Downloading CanvasKit...                                         1,107ms
Running "flutter pub get" in example...
Resolving dependencies... 
The current Dart SDK version is 2.19.4.

Because maplibre_gl_example requires SDK version >=3.0.0 <4.0.0, version solving failed.

To have a dart 3.x image, we need to move away from cirrusci/flutter, as it's deprecated. The recommended replacement is ghcr.io/cirruslabs/flutter

Cf message on cirrusci/flutter docker image: https://hub.docker.com/r/cirrusci/flutter

DEPRECATED
New releases available at https://github.com/cirruslabs/docker-images-flutter/pkgs/container/flutter

[m0nac0]

Thank you!