Fix warning generated by flutter pub publish --dry-run.

Package validation found the following potential issue:
* The Flutter constraint should not have an upper bound.
  In your pubspec.yaml the constraint is currently `>=1.17.0 <2.0.0`.

  You can replace that with just the lower bound: `>=1.17.0`.

  See https://dart.dev/go/flutter-upper-bound-deprecation

Package has 1 warning.

pubspec.yaml

@@ -5,7 +5,7 @@ repository: https://github.com/ably/ably-flutter
 
 environment:
   sdk: ">=2.6.0 <3.0.0"
-  flutter: ">=1.17.0 <2.0.0"
+  flutter: ">=1.17.0"
 
 dependencies:
   collection: ^1.14.13