Fix platformio.ini for MKS Robin (#13502)

platformio.ini

 # MKS Robin (STM32F103ZET6)
 #
 [env:mks_robin]
-platform      = ststm32
+platform      = ststm32@5.1.0
 framework     = arduino
 board         = genericSTM32F103ZE
 extra_scripts = buildroot/share/PlatformIO/scripts/mks_robin.py
 build_flags   = !python Marlin/src/HAL/HAL_STM32F1/STM32F1_flag_script.py
+ -DSTM32_XL_DENSITY 
   ${common.build_flags}
   -DSTM32_XL_DENSITY
 src_filter    = ${common.default_src_filter} +<src/HAL/HAL_STM32F1>