Key developments on June 20-21: * Ukraine strikes both sides of the occupied-Crimean Bridge in overnight drone attack, Zelensky says * Russia preparing 'new massive strike,' Zelensky warns * Zelensky mails revoked Polish medal back to Warsaw * Russian oil, gas sites in Crimea and Siberia hit by Ukrainian drones * At least 5 dead, 12 injured in Russian bombing in Zaporizhzhia Ukraine's army struck energy and military targets on both sides of the Crimean Bridge, Ukrainian President Volodymy