change default height above atmosphere orbit to be 5000 instead of 20000

if we already tell the user what the minimum value is, then we dont need to set them so high above the atmosphere, as this could just be confusing. 5000 is more than high enough to counteract principia shenanigans, while still allowing the user to be in a low orbit by default

Source/RP0/UI/KCT/GUI_Simulation.cs

@@ -344,7 +344,7 @@ private static double EnsureSafeMaxAltitude(double altitudeMeters, CelestialBody
 
         private static double GetDefaultAltitudeForBody(CelestialBody body)
         {
-            return body.atmosphere ? body.atmosphereDepth + 20010 : 20000;
+            return body.atmosphere ? body.atmosphereDepth + 5000 : 20000;
         }
     }
 }