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

abtink/openthread,jwhui/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread,abtink/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread,srickardti/openthread,srickardti/openthread,openthread/openthread,abtink/openthread,openthread/openthread,abtink/openthread