Some project maintenance, use `flutter_map` version `4.0.0` in example project

[josxha]

I wanted to upgrade your package to support flutter_map 4.0.0 but realized that there is no breaking change. So i did some small things instead:

• android example project maintenance
  • upgrade gradle to support Java 19 (https://docs.gradle.org/current/userguide/compatibility.html)
  • update targetSdkVersion to 33
  • remove deprecated android.enableR8
• (hopefully) fix the screenshot image on pub.dev once and for all by using the absolute url from github.
• remove deprecated lint invariant_booleans (https://github.com/dart-lang/linter/issues/3336)
• use version 4.0.0 of flutter_map in the example project and loosen dependency constraint (it was hard set to 3.0.0)
• increase minimum flutter_map version to ^4.0.0 and lower it back down to >=3.0.0 (woopsi)

[matthiasdittmer]

Thank you for the upgrades. I will do a new minor release soon.

I was notified of the new flutter_map release yesterday. The plugin does indeed work with with version 3.0.0 as well as the new 3.1.0 and 4.0.0 releases.

[matthiasdittmer]

Minor release 0.2.6 just done.

@josxha The image on pub.dev does work now. 👍