Bug#1140541: update debian/copyright

[Drew Parsons]

Package: vtk9
Version: 9.6.2+dfsg1-1exp1
Severity: normal

Andrew McMillan reports:

A significant number of authors are detected as not being acknowledged
in debian/copyright - please review the list in the full note and make
any adjustments you feel necessary.

License reconciliation suggests that there is some mismatch in which
licenses apply to which files, and that this aspect of the
debian/copyright file could also do with some clean up work when you
have time.

see https://dfsg-new-queue.debian.org/reviews/vtk9