Move Kossel Pro platform mesh a bit lower to prevent Z-fighting

As reported in #8296.

resources/definitions/anycubic_kossel_linear_plus.def.json

@@ -5,6 +5,7 @@
     "metadata": {
         "visible": true,
         "platform": "kossel_pro_build_platform.3mf",
+        "platform_offset": [0, -0.25, 0],
         "machine_extruder_trains": {
             "0": "anycubic_kossel_extruder_0"
         }