Add FairRoot licence files and drop the FairRoot find module

This adds the licence and origin notes for the imported FairRoot sources and removes the leftover FairRoot lookup.

- LICENSE and COPYRIGHT are copied verbatim from FairRoot v18.4.9-alice3, as the file headers refer to them.
- A README records the upstream version and what was changed.
- dependencies/FindFairRoot.cmake and the FairRoot package properties are removed, as nothing looks up FairRoot any more.

https://its.cern.ch/jira/browse/O2-7305

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

Detectors/3rdparty/FairRootBase/COPYRIGHT

@@ -0,0 +1,9 @@
+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: FairRoot
+Upstream-Contact: Mohammad Al-Turany <m.al-turany@gsi.de>
+Source: https://github.com/FairRootGroup/FairRoot
+
+Files: *
+Copyright: 2003-2019, GSI Helmholtzzentrum fuer Schwerionenforschung GmbH
+Copyright: 2003-2019, [see AUTHORS file]
+Copyright: 2003-2019, [see CONTRIBUTORS file]