Fix for dependency conflicts between Flutter versions

[ikurek]

This should close https://github.com/ably/ably-flutter/issues/394. Flutter 3.0.0 was released yesterday, and Ably SDK wasn't compatible with it. I've investigated the issues and here are the changes:

• Updated the minimum required version of Flutter for ably-flutter package from 2.2.3 to 2.5.0. This would usually be a breaking change, but for 2.2.3 the library wouldn't work anyway, because 2.2.3 includes Dart 2.13.4 and for package dependencies Dart 2.14.0 is required. Trying to build the package with 2.2.3 will fail due to dependency constraints.
• Set a minumum required Flutter version for the example app to 2.8.0. Previously it wasn't set at all, and due to its construction, example app won't work on any Flutter version below 2.8.0. Below this, it will fail on Android devices due to missing values in Android Manifest
• Added an override for test dependencies - it was required, because latest version of Flutter conflicts with test package. I've created an issue to investigate if it's possible to simplify test dependencies: https://github.com/ably/ably-flutter/issues/397

This should make the library compatible with all versions of Flutter from 2.5.0 to 3.0.0.