It has been over two years since RHEL-8 was released, and thus per the
platform build policy, we no longer need to support RHEL-7 as a build
target. This lets us increment the minimum required nettle version and
drop a lot of backwards compatibility code for 2.x series of nettle.
Per repology, current shipping versions are:
RHEL-8: 3.4.1
Debian Stretch: 3.3
Debian Buster: 3.4.1
openSUSE Leap 15.2: 3.4.1
Ubuntu LTS 18.04: 3.4
Ubuntu LTS 20.04: 3.5.1
FreeBSD: 3.7.2
Fedora 33: 3.5.1
Fedora 34: 3.7.2
OpenBSD: 3.7.2
macOS HomeBrew: 3.7.2
Debian Stretch has the oldest version and so 3.3 is the new minimum.
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>