box: fix upgrade script for _fk_constraint space

The set_system_triggers and erase routines in upgrade.lua did not
proceed actions for _fk_constraint space.

src/box/lua/upgrade.lua

@@ -73,6 +73,7 @@ local function set_system_triggers(val)
     box.space._collation:run_triggers(val)
     box.space._schema:run_triggers(val)
     box.space._cluster:run_triggers(val)
+    box.space._fk_constraint:run_triggers(val)
 end
 
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@@ -92,6 +93,7 @@ local function erase()
     truncate(box.space._trigger)
     truncate(box.space._schema)
     truncate(box.space._cluster)
+    truncate(box.space._fk_constraint)
 end
 
 local function create_sysview(source_id, target_id)