[doc] update license for alpaca.cow to standard Cowsay "GPLv1+ & Artistic 1.0"

This updates the `copyright` file to reflect @mai-space's application of expanded "GPLv1+ & Artistic 1.0" license grant to their `alpaca.cow` contribution, bringing its licensing in line with the original Cowsay licensing terms, per https://github.com/cowsay-org/cowsay/pull/33#issuecomment-2510206345.