Fix Creality RET6 env - RE (#19340)

platformio.ini

 [env:STM32F103RET6_creality]
 platform        = ${common_stm32f1.platform}
 extends         = common_stm32f1
-board           = genericSTM32F103RC
+board           = genericSTM32F103RE
 build_flags     = !python Marlin/src/HAL/STM32F1/build_flags.py
   ${common.build_flags} -std=gnu++14 -DSTM32_XL_DENSITY -DTEMP_TIMER_CHAN=4
 extra_scripts   = ${common.extra_scripts}