[in_app_purchase] update min Flutter version to 1.20.0

[cyanglaz]

The plugin's iOS folder has been removed due to federation. This is to support no iOS folder:

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.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool format)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• 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 or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[mvanbeusekom]

LGTM