Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

AK/Tests/TestJSON.cpp

@@ -61,9 +61,7 @@ TEST_CASE(load_form)
     widgets.for_each([&](const JsonValue& widget_value) {
         auto& widget_object = widget_value.as_object();
         auto widget_class = widget_object.get("class").as_string();
-        widget_object.for_each_member([&](auto& property_name, const JsonValue& property_value) {
-            (void)property_name;
-            (void)property_value;
+        widget_object.for_each_member([&]([[maybe_unused]] auto& property_name, [[maybe_unused]] const JsonValue& property_value) {
             //dbgprintf("Set property %s.%s to '%s'\n", widget_class.characters(), property_name.characters(), property_value.serialized().characters());
         });
     });