Set minimal flutter version to 1.20

[renefloor]

Probably we can set the minimal flutter version higher, but during publishing I got the following error:

pubspec.yaml allows Flutter SDK version prior to 1.20.0, which does not support having no `ios/` folder.
Please consider increasing the Flutter SDK requirement to ^1.20.0 or higher (environment.sdk.flutter) or create an `ios/` folder.

See https://flutter.dev/docs/development/packages-and-plugins/developing-packages#plugin

So with Flutter version set to 1.20 I could publish all packages.

Pre-launch Checklist

• I updated pubspec.yaml with an appropriate new version according to the [pub versioning philosophy].
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making if a test is possible.
• All existing and new tests are passing.