ui/spice: Require spice-server >= 0.14.0

Version 0.14.0 is now old enough to have made it into the major
distributions:

     Debian 11: 0.14.3
     RHEL-8: 0.14.3
     FreeBSD (ports): 0.15.0
     Fedora 35: 0.15.0
     Ubuntu 20.04: 0.14.2
     OpenSUSE Leap 15.3: 0.14.3

Requiring it lets us drop a number of version checks.  The next commit
will clean up some more.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230109190321.1056914-6-armbru@redhat.com>