Upgrade to Ubuntu 26.04 (Resolute Raccoon) in external repository

We'd like to upgrade everywhere we use Ubuntu in the external repository we use to publish pre-built versions of Sleeper, to use Ubuntu 26.04 instead.

Start by locating every Ubuntu reference in the external repository used to publish pre-built Sleeper versions, then inspect the workflows or configuration entries containing those references. Update the Ubuntu version consistently to 26.04 and verify that the repository's pre-built publishing process still completes successfully.

Written by the indexing model from the issue text.