KYIV, Ukraine — Ukrainian President Volodymyr Zelenskyy said on Sunday he had decided to dismiss Defense Minister Oleksii Reznikov from his post and would ask parliament this week to replace him with Rustem Umerov, head of Ukraine’s main privatization fund. The announcement, made in his nightly video address to the nation, sets the stage for the biggest shakeup of Ukraine’s defense establishment during the war launched by Russia in February 2022.