Enable `propose_downstream` to automate new releases

[psss]

Let's simplify new releases by enabling propose_downstream in Packit.

[lachmanfrantisek]

https://forge.fedoraproject.org/ai-ml/skills-library/src/branch/main/skills/packit-pull-from-upstream-onboarding/SKILL.md
Maybe it will come in handy for someone...