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/base/net/dnsupdater.cpp

@@ -153,7 +153,7 @@ QString DNSUpdater::getUpdateUrl() const
         break;
     default:
         qWarning() << "Unrecognized Dynamic DNS service!";
-        Q_ASSERT(false);
+        Q_UNREACHABLE();
         break;
     }
     url.setPath(u"/nic/update"_s);
@@ -305,7 +305,7 @@ QUrl DNSUpdater::getRegistrationUrl(const DNS::Service service)
     case DNS::Service::NoIP:
         return {u"https://www.noip.com/remote-access"_s};
     default:
-        Q_ASSERT(false);
+        Q_UNREACHABLE();
         break;
     }
     return {};