Remove copyright & years from all code, except LICENSE.md etc

I'm browsing for a small change made to a template, and I'm navigating through a bunch of NFC changes like 2a73d3b0 and 6b83d5bd. (Clicking on those commits may even WSoD Gitlab, so here's 2a73d3b on Github.)

Credit to those doing the busywork, but those changes contribute little to the software while adding noise to the commit history of every file in CiviCRM.

The commit messages are fairly inconsistent over time, so someone browsing history can't be 100% sure that any given commit is safe to ignore.

I say it's time to stop changing hundreds of^W^W thousands of (?!) files over and over just to update the copyright & CiviCRM version.

Let's make one final commit and remove them all, removing those comments in favour of references to licensing via COPYRIGHT.md (copyright year and owner), LICENSE.md (contents of the AGPL), and https://civicrm.org/licensing instead.

No shortage of other projects demonstrating this - I don't think it's legally contentious based on the number of projects using that approach.