update to flutter_map 3.0

[josxha]

Version 3.0 of flutter_map introduces breaking changes that require implementation changes to plugins.

• https://pub.dev/packages/flutter_map/changelog
• https://docs.fleaflet.dev/migration/to-v3.0.0

Here is a list with the changes in this pull request:

• upgrade packages
• bump minimum dart version to 2.17 (same as flutter_map)
• minimum version of flutter_map is 3.0, backwards support is not possible because of the new layering system
• use the naming convention of flutter_map
  • MapPluginLatLonGridOptions is now LatLonGridLayerOptions
  • MapPluginLatLonGridOptions is now LatLonGridLayerOptions
• use the new layering system (every layer is now a Widget)
• update the implementation of the example app
• minor performance improvements by using final and const
• bump gradle to 7.0 to get rid of a warning while building
• use custom app name and package id on android to prevent collision with other example apps
• update README.md
  • limit size of example screenshot
  • update example code

[josxha]

@matthiasdittmer could you take a look on this pr at some point please?

[matthiasdittmer]

@josxha I missed the first notification. I will review your changes and merge them soon.