Make KEEPALIVE_STATE a scoped auto-restore (#14446)

Marlin/src/feature/bedlevel/ubl/ubl_G29.cpp

Marlin/src/feature/pause.cpp

Marlin/src/feature/prusa_MMU2/mmu2.cpp

Marlin/src/feature/prusa_MMU2/mmu2.h

Marlin/src/gcode/bedlevel/abl/G29.cpp

Marlin/src/gcode/config/M43.cpp

Marlin/src/gcode/gcode.cpp

Marlin/src/gcode/gcode.h

Marlin/src/gcode/lcd/M0_M1.cpp

Marlin/src/gcode/temperature/M303.cpp

Marlin/src/lcd/menu/menu_delta_calibrate.cpp

Marlin/src/module/probe.cpp

Marlin/src/module/temperature.cpp