Complex sentence: On 12 April 2015 Soyuz-U was claimed obsolete, its production stopped already and the usage will end when all stored vehicles will be launched, mostly with "Progress" cargo ships.
Split into simpler sentences: On 12 April 2015 Soyuz-U was declared to be obsolete. Its production has been stopped, and usage of the rocket will end when all stored vehicles have been launched, mostly with Progress cargo ships.