[mythtv] COPYING & AUTHORS files

[Gary Buhrmaster]

I note that on the MythTV trac page that the GPLv3
logo is shown.  However, the file COPYING in the
base mythtv directory is still GPLv2.  While I do not
recall an explicit promotion to GPLv3 (allowed,
of course, in GPLv2), it probably would be good to
make things consistent.  I will also note that the
AUTHORS file is not aging well.  And the
UPGRADING file probably just needs a pointer
to the wiki (or deleted).

Perhaps time to update some of those files?