Update copyright year and automatically check modified files

[nategraf]

This PR updates the copyright year to 2023 in .licenserc.json, and in all files that have been modified since the start of the year.

Additionally, this PR includes a new copyright check command that ensures a PR will require an updated copyright on any files modified in the PR.

Opened this PR after running into the copyright check in #330

[nategraf]

This PR is currently trying to use a git command to figure out which files have been touched since the beginning of the year, but there is something about the way GH Actions checks out repos that is preventing that from working. I'll spend a few more minutes looking into this on Monday and, if I can't get it to work, opt for a simpler solution.

[flaub]

Thanks!!