Support wildcard (`major.minor`) SDK version specifiers

[parlough]

Now that setup-dart has wildcard version specifiers, mono_repo should be updated to support them. So we don't have to regenerate files to use each latest patch SDK version.

It should:

1. Accept wildcard versions such as 3.1 in the list of SDKs
2. Accept a new pubspec-minor (or maybe pubspec-wildcard) version similar to pubspec that uses the SDK lowest bound in the pubspec, but without the patch version.