Allow tweaking try-unload feedrates

It looks like MMU2_VERIFY_LOAD_TO_NOZZLE_FEED_RATE can get much higher that the original 20mm/s.
50mm/s is the default now, can be tweaked further.

Firmware/mmu2.cpp

@@ -250,9 +250,9 @@ bool MMU2::VerifyFilamentEnteredPTFE()
     // MMU has finished its load, push the filament further by some defined constant length
     // If the filament sensor reads 0 at any moment, then report FAILURE
     current_position[E_AXIS] += MMU2_EXTRUDER_PTFE_LENGTH + MMU2_EXTRUDER_HEATBREAK_LENGTH - (logic.ExtraLoadDistance() - MMU2_FILAMENT_SENSOR_POSITION);
-    plan_buffer_line_curposXYZE(MMU2_LOAD_TO_NOZZLE_FEED_RATE);
+    plan_buffer_line_curposXYZE(MMU2_VERIFY_LOAD_TO_NOZZLE_FEED_RATE);
     current_position[E_AXIS] -= (MMU2_EXTRUDER_PTFE_LENGTH + MMU2_EXTRUDER_HEATBREAK_LENGTH - (logic.ExtraLoadDistance() - MMU2_FILAMENT_SENSOR_POSITION));
-    plan_buffer_line_curposXYZE(MMU2_LOAD_TO_NOZZLE_FEED_RATE);
+    plan_buffer_line_curposXYZE(MMU2_VERIFY_LOAD_TO_NOZZLE_FEED_RATE);
 
     while(blocks_queued())
     {

Firmware/mmu2/variants/config_MMU2S.h

@@ -33,6 +33,8 @@ static constexpr float MMU2_EXTRUDER_HEATBREAK_LENGTH  = 17.7f; // mm
 static constexpr float MMU2_LOAD_TO_NOZZLE_FEED_RATE = 20.0F; // mm/s
 static constexpr float MMU2_UNLOAD_TO_FINDA_FEED_RATE = 120.0F; // mm/s
 
+static constexpr float MMU2_VERIFY_LOAD_TO_NOZZLE_FEED_RATE = 50.0F; // mm/s
+
 // The first the MMU does is initialise its axis. Meanwhile the E-motor will unload 20mm of filament in approx. 1 second.
 static constexpr float MMU2_RETRY_UNLOAD_TO_FINDA_LENGTH = 80.0f; // mm
 static constexpr float MMU2_RETRY_UNLOAD_TO_FINDA_FEED_RATE = 80.0f; // mm/s