Fix cosmetic variations in copyrights and license files

[stuartmorgan-g]

This does a few things:

• Replaces "the Flutter project authors" with the repo-standard version "The Flutter Authors"
  • Updates the license check not to allow "the Flutter project authors" in the future
• Fixes a few minor cosmetic variations that had crept back into LICENSE files since my
  mass-standardization of those files.
  • Updates the license check to validate those to prevent such drift in the future.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[mvanbeusekom]

LGTM!