build: bump minimum glib version to 2.68

build: bump minimum glib version to 2.68

We removed support for Debian 11 which only had 2.66.8.
Next stop: 2.72 after we drop Ubuntu 22.04

For libvirt, the update to the 2.68 GLib release:
* introduces g_string_replace
* deprecates g_memdup in favor of g_memdup2
* removes the need for some warning workarounds
* deprecates g_time_zone_new in favor of g_time_zone_new_identifier
  which returns NULL on error instead of returning UTC

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