Use fvm/flutter `flavors`

[bonomat]

This PR: https://github.com/get10101/10101/pull/2306 uses a simple solution to overwrite the flutter version for a specific module but a better approach would be to use flavors

https://fvm.app/documentation/guides/project-flavors

Originally posted by @bonomat in https://github.com/get10101/10101/issues/2306#issuecomment-2015908345

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