Update to Freeciv server 57c670fc31
Update to Freeciv server 57c670fc31

Freeciv commit message:
Drop DEV_SAVE_COMPAT from version < 2.91

See hrm Feature #664190

(Signed-off-by line removed from Freeciv commit message to avoid confusion)

andreasrosdal/freeciv-web,andreasrosdal/freeciv-web,andreasrosdal/freeciv-web,andreasrosdal/freeciv-web,andreasrosdal/freeciv-web