🔨 Fix marlin_STM32H723VG mcu value (#25991)

Followup to #25921

buildroot/share/PlatformIO/boards/marlin_STM32H723VG.json

@@ -4,7 +4,7 @@
     "cpu": "cortex-m7",
     "extra_flags": "-DSTM32H7xx -DSTM32H723xx",
     "f_cpu": "550000000L",
-    "mcu": "stm32h723vzt6",
+    "mcu": "stm32h723vgt6",
     "product_line": "STM32H723xx",
     "variant": "MARLIN_H723VG"
   },