crypto: bump min gnutls to 3.5.18, dropping RHEL-7 support

crypto: bump min gnutls to 3.5.18, dropping RHEL-7 support

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 gnutls version

Per repology, current shipping versions are:

             RHEL-8: 3.6.14
      Debian Buster: 3.6.7
 openSUSE Leap 15.2: 3.6.7
   Ubuntu LTS 18.04: 3.5.18
   Ubuntu LTS 20.04: 3.6.13
            FreeBSD: 3.6.15
          Fedora 33: 3.6.16
          Fedora 34: 3.7.1
            OpenBSD: 3.6.15
     macOS HomeBrew: 3.6.15

Ubuntu LTS 18.04 has the oldest version and so 3.5.18 is the new minimum.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210514120415.1368922-7-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
[thuth: rebased to use .gitlab-ci.d/buildtest.yml]
Signed-off-by: Thomas Huth <thuth@redhat.com>