Migrate project to Dart 3

[orestesgaolin]

In an effort to bring it to runnable state here's a chunky PR that brings the following changes:

• Migrate to Dart 3, update version constraints to require Flutter >=3.3.0 and SDK >=3.0.0
• Migrate Android project to AGP8, set target and compile SDK to 34
• Updated gitignore to keep lock files and Podfile in example app
• Updated analysis_options.yaml
• Cosmetic updates to example app
• Set iOS deployment target to iOS 14
• Set Android min SDK to 21

iOS working on physical device with locally deployed backend:

https://github.com/fluttercommunity/flutter_uploader/assets/16854239/7e2de04c-19cf-4d0e-9488-f875f85421d1

Tested on Android as well:

https://github.com/fluttercommunity/flutter_uploader/assets/16854239/b2c336f6-e9f6-41ee-a676-3300f9a705d3

Some issues still pending:

• upload progress is not emitted correctly for bigger uploads
• failed upload is not handled correctly for bigger uploads

Tests passing locally

❯ flutter test
00:01 +11: All tests passed!

Closes #261 Closes #255 Closes #235 Closes #260 Closes #258 Closes #249