update dependency of tuple package from ^1.0.2 to ^2.0.0

[supermuesli]

tuple: ^1.0.2 is causing dependency conflicts in my application which can be solved by updating to tuple: ^2.0.0.
The changelogs of the tuple package suggest that there were no breaking changes, so I decided to updated the package in flutter_map to tuple: ^2.0.0.

[kengu]

@supermuesli PR #851 will resolve this when all dependencies have a null-safe version available. Closing.