Remove authors field from pubspec once Dart 2.7.0 is released

[evanweible-wf]

There's an upcoming change in 2.7-dev where it warns about the author[s] field if the package has a verified publisher on pub. To workaround that, in #333 we changed up our CI so that pub publish --dry-run only runs on the stable channel of Dart.

Once Dart 2.7.0 is released, we can remove that field in our pubspec.yaml and pub publish --dry-run should start passing. We'll then need to publish new releases with >=2.7.0.