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.

Services/DHCPClient/main.cpp

@@ -51,11 +51,8 @@ static MACAddress mac_from_string(const String& str)
     };
 }
 
-int main(int argc, char** argv)
+int main([[maybe_unused]] int argc, [[maybe_unused]] char** argv)
 {
-    (void)argc;
-    (void)argv;
-
     if (pledge("stdio unix inet cpath rpath fattr", nullptr) < 0) {
         perror("pledge");
         return 1;