Use proper macro for unreachable switch cases

Those are the `default` cases which are not expected to hit (nor reachable) normally.

When the code is compiled with release mode and it reaches `Q_UNREACHABLE()`, it becomes
undefined behavior. So it rely on the developers to catch the errors in debug mode.
The upside of this is that the `switch` statement will be more optimized than not using it.
This also means the statements after `Q_UNREACHABLE()` isn't important. It allow anything to
preserve the intention of the code.

This macro is preferred over C++23 `std::unreachable` because it will automatically insert a
`Q_ASSERT(false)` with it.

PR #21752.

src/webui/api/synccontroller.cpp

@@ -288,7 +288,7 @@ namespace
                     }
                     break;
                 default:
-                    Q_ASSERT(false);
+                    Q_UNREACHABLE();
                     break;
                 }
             }