Bump required GLib version to 2.56

g_date_time_new_from_iso8601() is available since GLib 2.56
2 jobs for !13 with iso8601 in 1 minute and 19 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Build
passed #9799
build

00:00:31

 
  Distcheck
passed #9800
distcheck

00:00:47