Change the copyright holder to Coffey Labs

Two scripts and the README's licence section. LINUXexpert.org is being retired
as a site and is no longer where this work lives.

The clone URL in the README still says github.com/LINUXexpert-org/ubuntu2mint,
because that is the repository's actual path. The GitHub organisation slug and
the copyright holder are different things and only one of them is changing.

LICENSE untouched -- its "<name of author>" lines are GPL boilerplate, and the
FSF's copyright on the licence text is not ours to edit.

ubuntu-to-mint-convert-v3.sh

@@ -28,7 +28,7 @@
 # accept that the most reliable rollback is “restore from backup.”
 # -----------------------------------------------------------------------------
 #
-# Copyright (C) 2026 LINUXexpert.org
+# Copyright (C) 2026 Coffey Labs
 #
 # This program is free software: you can redistribute it and/or modify it
 # under the terms of the GNU General Public License as published by the