Check requirements and update for Flutter 3.0.0

[ikurek]

Flutter 3 was just released, and {{ably-flutter}} is not compatible with it. The SDK constraint in doesn't allow versions above 2:

ably-flutter/pubspec.yaml

Line 7 in 45f6fa4

sdk: ">=2.12.0 <3.0.0"

And even after it's changed to support version 3, the {{pub get}} command fails with this output:

{code}
[ably-flutter] flutter pub get
Running "flutter pub get" in ably-flutter...
Because test 1.19.5 depends on test_api 0.4.8 and every version of flutter_test from sdk depends on test_api 0.4.9, test 1.19.5 is incompatible with flutter_test from sdk.
So, because ably_flutter depends on both flutter_test from sdk and test 1.19.5, version solving failed.
pub get failed (1; So, because ably_flutter depends on both flutter_test from sdk and test 1.19.5, version solving failed.)
{code}
We should update required dependencies and release a fix, to allow builds on latest Flutter versions