SanityCheck: No Adafruit NeoPixel for STM32F1 (#18833)

Marlin/src/HAL/STM32F1/inc/SanityCheck.h

 #elif ENABLED(SERIAL_STATS_DROPPED_RX)
   #error "SERIAL_STATS_DROPPED_RX is not supported on this platform."
 #endif
+
+#if ENABLED(NEOPIXEL_LED)
+  #error "NEOPIXEL_LED (Adafruit NeoPixel) is not supported for HAL/STM32F1. Comment out this line to proceed at your own risk!"
+#endif