Remove use of legacy keycode (qmk#18281)

Author: zvecr

quantum/process_keycode/process_auto_shift.h

@@ -32,7 +32,7 @@
 #define AUTO_SHIFT_SPECIAL          \
              KC_TAB:                \
         case KC_MINUS ... KC_SLASH: \
-        case KC_NONUS_BSLASH
+        case KC_NONUS_BACKSLASH
 // clang-format on
 
 bool process_auto_shift(uint16_t keycode, keyrecord_t *record);