Preserve brightness in EEPROM validate (#19485)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>

Marlin/src/module/settings.cpp

       //
       {
         _FIELD_TEST(lcd_contrast);
-
         int16_t lcd_contrast;
         EEPROM_READ(lcd_contrast);
-        TERN_(HAS_LCD_CONTRAST, ui.set_contrast(lcd_contrast));
+        if (!validating) {
+          TERN_(HAS_LCD_CONTRAST, ui.set_contrast(lcd_contrast));
+        }
       }
 
       //