[dua] remove DUA address if network data no longer contains DUA prefix (#8874)

Fix for the following bug:

Leader/Parent (OTBR) is reset (taken off the network), leading to its
MTD child being detached. When the parent is brought back on the
network without configuring the previous DUA prefix, the MTD asserts
complaining that its DUA prefix is not valid anymore.

1. We don't assert anymore in this situation, rather the MTD just
   won't include the DUA address in its address registration TLV.

2. More importantly, we remove a "cached" DUA address if we find that
   the network leader is no longer configured with a DUA prefix.

src/core/thread/dua_manager.cpp

@@ -305,6 +305,19 @@ void DuaManager::HandleNotifierEvents(Events aEvents)
 {
     Mle::MleRouter &mle = Get<Mle::MleRouter>();
 
+#if OPENTHREAD_CONFIG_DUA_ENABLE
+    if (aEvents.Contains(kEventThreadNetdataChanged))
+    {
+        Lowpan::Context context;
+        // Remove a stale DUA address if any.
+        if (Get<ThreadNetif>().HasUnicastAddress(Get<DuaManager>().GetDomainUnicastAddress()) &&
+            (Get<NetworkData::Leader>().GetContext(Get<DuaManager>().GetDomainUnicastAddress(), context) != kErrorNone))
+        {
+            RemoveDomainUnicastAddress();
+        }
+    }
+#endif
+
     VerifyOrExit(mle.IsAttached(), mDelay.mValue = 0);
 
     if (aEvents.Contains(kEventThreadRoleChanged))