configure: bump min required CLang to 6.0 / XCode 10.0

configure: bump min required CLang to 6.0 / XCode 10.0

Several distros have been dropped since the last time we bumped the
minimum required CLang version.

Per repology, currently shipping versions are:

             RHEL-8: 10.0.1
      Debian Buster: 7.0.1
 openSUSE Leap 15.2: 9.0.1
   Ubuntu LTS 18.04: 6.0.0
   Ubuntu LTS 20.04: 10.0.0
         FreeBSD 12: 8.0.1
          Fedora 33: 11.0.0
          Fedora 34: 11.1.0

With this list Ubuntu LTS 18.04 is the constraint at 6.0.0

An LLVM version of 6.0.0 corresponds to macOS XCode version of 10.0
which dates from Sept 2018.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>