set minimum safety threshold to 60 mg/dL

- Changes default of minimum safety threshold to `60` from `65` to match standard oref.
- Changes name of setting from `Threshold Setting (mg/dL)` to `Minimum Safety Threshold (mg/dL)`.
- Updates tooltip to improve clarity.

FreeAPS/Resources/json/defaults/preferences.json

@@ -49,6 +49,6 @@
   "tddAdjBasal" : false,
   "enableSMB_high_bg" : false,
   "enableSMB_high_bg_target" : 110,
-  "threshold_setting" : 65,
+  "threshold_setting" : 60,
   "updateInterval" : 20
 }