🔧 More suppressible warnings

[thisiskeithb]

Description

• Suppressible LCD contrast warning: Users don't need to know or care about something their LCD doesn't support, so let's make it easier to silence the warning. In fact, this entire warning could be removed, but I left it here anyway.
• Suppressible Z_SAFE_HOMING warning: Homing with a probe to the right of a hotend usually doesn't require homing to the center of the bed if homing to X min (i.e.: Prusa MK2/3, Ender-3, etc.), so let's make it easier to silence the warning.
• Suppressible user feedback warning: External G-code senders that communicate via serial can handle / remap G-codes as they are processed (like with BTT's TFT firmware), so it'd be nice to have the option to silence this warning on Marlin's side.

Benefits

Users won't have to venture into Warnings.cpp to override frequently encountered warnings and this is more in line with how we allow other warnings to be disabled without diving into core Marlin files.