[routing-manager] PublishExternalRoute() to return success on kErrorAlready (#7715)

This commit changes `RoutingManager::PublishExternalRoute()` method
to return `kErrorAlready` as `kErrorNone` (i.e., if the prefix to be
published was previously published). This commit keeps the logging
the same (i.e. we still log on `Already` error so we can see when/if
this happens).

src/core/border_router/routing_manager.cpp

@@ -565,7 +565,7 @@ Error RoutingManager::PublishExternalRoute(const Ip6::Prefix &aPrefix, RoutePref
         LogWarn("Failed to publish external route %s: %s", aPrefix.ToString().AsCString(), ErrorToString(error));
     }
 
-    return error;
+    return (error == kErrorAlready) ? kErrorNone : error;
 }
 
 void RoutingManager::UnpublishExternalRoute(const Ip6::Prefix &aPrefix)