Use M256 (set brightness) instead of M250 (set contrast)

[andrivet]

Bug Report

• You have checked that you are not creating a duplicate of an existing (maybe closed) issue (use the search)
• You have tested with the latest stable version of ADVi3++
• You are able to reproduce the bug
• You are able to describe step by step the process to reproduce the bug

Description of the bug

• Exact model of your printer: Wanhao Duplicator i3 Plus
• ADVi3++ exact version (Mainboard): 5.0.1
• Printing using SD card or USB: n/a
• Do you use a BLTouch?: n/a
• Expected behaviour: M256 should set the brightness of the LCD panel
• Actual behaviour: Instead, you have to use M250
• Steps to reproduce:
  • M250 C20 set the brightness
  • M256 B20 is not defined (error)

Note: M256 was added recently (MarlinFirmware/Marlin#22478) so not really a bug, but M256 is better than M250.

[andrivet]

Implemented in version 5.1