fix: update min Dart SDK version

[mrverdant13]

Description

When creating a new project of type dart_pkg, the pubspec.yaml includes the following environment setup:

environment:
  sdk: ">=2.12.0 <3.0.0"

Seems like something trivial, but it is sometimes annoying to update nested packages after their creation.

Steps To Reproduce

1. Run very_good create --project-name my_pkg --org-name com.my.org -t dart_pkg ./my_pkg.
2. Go to ./my_pkg/pubspec.yaml.
3. See the environment config field.

Expected Behavior

The expected min Dart SDK is expected to be 2.13.0 as follows:

environment:
  sdk: ">=2.13.0 <3.0.0"

Additional Context

CLI version: very_good_cli 0.3.2