Standardize copyright year

[stuartmorgan-g]

Standardizes all first-party copyrights on a single year, as is done in flutter/flutter and flutter/engine. All code now uses 2013, which is the earliest year that was in any existing copyright notice.

The script checks now enforce the exact format of first-party licenses and copyrights.

Fixes flutter/flutter#78448

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.

[stuartmorgan-g]

The first commit has the script changes; all subsequent commits are the mechanical replacements.

[ditman]

RSLGTM!

Merge conflicts for everyone!! :P