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.

Userland/sort.cpp

@@ -32,16 +32,13 @@
 #include <stdlib.h>
 #include <string.h>
 
-int main(int argc, char** argv)
+int main([[maybe_unused]] int argc, [[maybe_unused]] char** argv)
 {
     if (pledge("stdio", nullptr) > 0) {
         perror("pledge");
         return 1;
     }
 
-    UNUSED_PARAM(argc);
-    UNUSED_PARAM(argv);
-
     Vector<String> lines;
 
     for (;;) {