calibration params: avoid using NaN as default

- NaN is not supported by JSON, and leads to parsing failure in QGC.
 - fixes https://github.com/PX4/PX4-Autopilot/issues/18095

src/modules/sensors/sensor_params_gyro1.c

@@ -145,8 +145,9 @@ PARAM_DEFINE_FLOAT(CAL_GYRO1_ZOFF, 0.0f);
  *
  * Temperature during last calibration.
  *
+ * @unit celcius
  * @category system
  * @group Sensor Calibration
  * @volatile
  */
-PARAM_DEFINE_FLOAT(CAL_GYRO1_TEMP, NAN);
+PARAM_DEFINE_FLOAT(CAL_GYRO1_TEMP, -1000.f);