meson: Require gnutls-3.6.0 or newer

meson: Require gnutls-3.6.0 or newer

Released almost 5 years ago, gnutls-3.6.0 brings some important
features (which are utilized in next commit). Hence, require that
version at least.

Per repology, currently shipped versions are:

                 RHEL-8: 3.6.16
                 RHEL-9: 3.7.3
              Debian 11: 3.7.1
              Debian 12: 3.7.6
     openSUSE Leap 15.3: 3.6.7
       Ubuntu LTS 20.04: 3.6.13
       Ubuntu LTS 22.04: 3.7.3
             FreeBSD 12: 3.7.6
              Fedora 34: 3.7.4
              Fedora 35: 3.7.6

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>