Extend Heater Overshoot Options (#21273)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/Configuration.h

 #define BED_MAXTEMP      150
 #define CHAMBER_MAXTEMP  60
 
+/**
+ * Thermal Overshoot
+ * During heatup (and printing) the temperature can often "overshoot" the target by many degrees
+ * (especially before PID tuning). Setting the target temperature too close to MAXTEMP guarantees
+ * a MAXTEMP shutdown! Use these values to forbid temperatures being set too close to MAXTEMP.
+ */
+#define HOTEND_OVERSHOOT 15   // (°C) Forbid temperatures over MAXTEMP - OVERSHOOT
+#define BED_OVERSHOOT    10   // (°C) Forbid temperatures over MAXTEMP - OVERSHOOT
+
 //===========================================================================
 //============================= PID Settings ================================
 //===========================================================================