ci: Update to Ubuntu 24.04 for Debian Testing builds

[jayaddison]

Resolves #654.

[jayaddison]

A patch has been posted to resolve this problem in jammy via the backport process, so this distribution-level upgrade may not be required.

[jayaddison]

A patch has been posted to resolve this problem in jammy via the backport process, so this distribution-level upgrade may not be required.

Note / clarification: that patch hasn't reached jammy yet, but if+when it does, then the seccomp unconfined change could be reverted, and this pull request could be closed.

[jayaddison]

The patch hasn't reached the latest 22.04.5 release of jammy, so on balance, I think I'd prefer to move this back into ready-for-review status (which if I remember correctly, implies mergable). I'm not sure whether it's better to merge, or wait for a subsequent jammy update, or to continue on as-is with the existing unconfined behaviour -- but doing this should provide the most options.

[ximion]

Nice, thanks!