update Cactus COPYRIGHT file to LGPL

Cactus originally used GPL with a spelled out exception that linking is exempt from the GPL copyleft, ie the same exception that LGPL used. Some time ago (around 2015, see eg commit logs in cactus* repos) we switched to explicitly using LGPL and updated the README files. Cactus itself however was forgotten (has no README file, but has COPYRIGHT). Pull request https://bitbucket.org/cactuscode/cactus/pull-requests/144 addresses this.

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