Don't test under 1.21.1 (dart-lang/pub_semver#33)

Context: https://github.com/dart-lang/pub_semver/commit/c8859a59688e71fb9d3a2a966340fbfaf45f6239#r29835939
1 file changed
tree: 3ae8ff01e79ecc4480b593683b28b208e4be7f4e
  1. pkgs/