As part of Org Mode’s migration to a new server, its ELPA repository has also moved. It used to be part of the GNU ELPA repository but is no longer housed there. Instead, Org now has its own repository. If you like to upgrade Org with the package system, you should add the new repository to your init.el.
To do that, simply add
(add-to-list 'package-archives '("org" . "https://orgmode.org/elpa/") t)
to your init.el.