Redmine

GitHub is gradually moving ubuntu-latest from Ubuntu 24.04 to Ubuntu 26.04. The current CI setup will fail when this change reaches the repository. This patch prepares the workflows in advance to avoid disrupting development.

• Use ubuntu-26.04 explicitly to run CI on the new environment before the automatic migration.
• Remove the ImageMagick 6 PDF policy override. Ubuntu 26.04 uses ImageMagick 7, whose default policy already allows PDF processing.
• Switch back to ubuntu-latest in a follow-up patch once the migration is complete. This is not urgent, as the workflows will already be running on Ubuntu 26.04.