Clarify Cowsay licensing situation

There are some questions about the exact state of the licensing situation for cowsay. And I think my documentation for it is currently a bit wrong, or at least incomplete. Sort this out. ## Progress Work is on the [`licentious` branch](https://github.com/cowsay-org/cowsay/tree/licentious). So far, it's just me doing some research and writing notes. As of December 2024, I now think the situation boils down to "original cowsay was dual-licensed GPL 1+ and Artistic License, with maybe a few specific cows under different licenses, and third-party contributions to cowsay-org since 2016 are just GPLv3". As of November 30, 2024, I re-licensed all my own contributions to Cowsay to expand the license grant to be "GNU GPL 1 or later or Artistic License 1.0" instead of just GPLv3. That's consistent with what I think the original licensing situation was, and gives good compatibility with future changes to the licensing or our understanding of it. I've started asking other contributors to do the same.