[webview_flutter] Remove the upper bound from the Flutter SDK constraint (#2517)

As the Flutter SDK does not follow semver, an upper bound has no semantic meaning.

Using this plugin as a canary to check if anything breaks when we remove this bound.

packages/webview_flutter/pubspec.yaml

diff --git a/packages/webview_flutter/pubspec.yaml b/packages/webview_flutter/pubspec.yaml
index 24a8eed..97d143a 100644
--- a/packages/webview_flutter/pubspec.yaml
+++ b/packages/webview_flutter/pubspec.yaml
@@ -1,11 +1,11 @@
 name: webview_flutter
 description: A Flutter plugin that provides a WebView widget on Android and iOS.
-version: 0.3.19+6
+version: 0.3.19+7
 homepage: https://github.com/flutter/plugins/tree/master/packages/webview_flutter
 
 environment:
   sdk: ">=2.0.0-dev.68.0 <3.0.0"
-  flutter: ">=1.12.13+hotfix.5 <2.0.0"
+  flutter: ">=1.12.13+hotfix.5"
 
 dependencies:
   flutter: