[routing-manager] fix build and test failures (#7707)
[routing-manager] fix build and test failures (#7707)

`RemoveExternalRoute` -> `UnpublishExternalRoute`

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