dns/knot2: upgrade to 2.9.8 and add deprecation advice to use knot3.x

PR: 249367
Submitted by: Leo Vandewoestijne <freebsd@dns.company> (maintainer)
Reviewed by: Chris Hutchinson <portmaster@bsdforge.com>
Relnotes: https://www.knot-dns.cz/2020-10-09-version-297.html

		https://www.knot-dns.cz/2020-12-14-version-298.html