vim: update obsolete comment

vim: update obsolete comment

vim 8.3 has been out for a long time, so this comment is obsolete.
However we still need UPSTREAM_VERSION_UNKNOWN, since we ignore
the last digit of the upstream version number.

Test result:
$ devtool check-upgrade-status vim
  ...
  INFO: vim                       9.0.1592        UNKNOWN         Tom Rini <trini@konsulko.com> c0370529c027abc5b1698d53fcfb8c02a0c515da

(From OE-Core rev: 65f5de85c3f488136d1ec2b1f7fe8d8426d6c5b3)

Signed-off-by: Etienne Cordonnier <ecordonnier@snap.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>