configure: bump min required glib version to 2.56

configure: bump min required glib version to 2.56

The glib version was not previously constrained by RHEL-7 since it
rebases fairly often. Instead SLES 12 and Ubuntu 16.04 were the
constraints in 00f2cfbbec63fb6f5a7789797a62ccedd22466ea. Both of
these are old enough that they are outside our platform support
matrix now.

Per repology, current shipping versions are:

             RHEL-8: 2.56.4
      Debian Buster: 2.58.3
 openSUSE Leap 15.2: 2.62.6
   Ubuntu LTS 18.04: 2.56.4
   Ubuntu LTS 20.04: 2.64.6
            FreeBSD: 2.66.7
          Fedora 33: 2.66.8
          Fedora 34: 2.68.1
            OpenBSD: 2.68.1
     macOS HomeBrew: 2.68.1

Thus Ubuntu LTS 18.04 / RHEL-8 are the constraint for GLib version
at 2.56

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210514120415.1368922-11-berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>