databases/db6: sunset (expire) port in half a year's time

Oracle have hidden newer versions behind a registration-only download,
so we cannot even update to 6.2.38. Oracle haven't responded to an
inquiry to that extent.

Apparently, the port isn't very popular, no port in INDEX-12 depends on it,
users will rather use db5 (perhaps due to the more permissive license),
so deprecate db6.