Unclear how to specify Dart version

I'm the developer behind Feeel.

I'm trying to release a new version, but the build is failing because a dependency I use, url_launcher 6.0.12, "requires SDK version >=2.14.0 <3.0.0". The SDK being referred to here seems to be the Dart SDK.

The Feeel yml file specifies Flutter v2.5.3 for building the latest version of the app. The Dart SDK is bundled within the Flutter SDK, so I assumed that Dart SDK 2.14.4, which is included with Flutter 2.5.3, would be used. Instead, however, it seems version 2.13.0 of the Dart SDK is used.

Why does this happen and how can I specify the correct Dart SDK version to be used?