Increase min android version to Android 7

[CodeDoctorDE]

Starting with Butterfly 2.4.0-beta.3, the minSdkVersion gets increased from 23 to 24 since Flutter 3.35 depend on this.
This means, you need Android Nougat (Android 7) or higher to run the app.

To run the newer version of the app, I recommend:

• Updating to a newer android version (if it is possible)
• Downloading a custom rom (like lineageos)
• Using the web version of Butterfly

See #780 for the previous android version bump.

See https://en.wikipedia.org/wiki/Android_version_history for reference.

Butterfly Adonis Blue (2.3) will keep supporting Android 6. It will be supported until the big Butterfly Black Hairstreak (2.4) update gets out (I'm planning the release in 2 months).