It has newer packages than Debian.
This is not quite true. They have overlapping release cycles. A new Debian release will ship frozen versions of the latest packages, causing it to have newer packages than most ubuntu releases. Then the new ubuntu release comes out, with and it has newer packages. Ubuntu doesn't universally newer packages than debian. The difference is that Debian ONLY does security updates, and doesn't do feature updates or even bugfixes over it's lifespan. Ubuntu, on the other hand, does ship feature updates and bug fixes, incrementing the package version as they go over the lifespan of an Ubuntu release.
Comparing the bash versions of the latest ubuntu stable version versus the current debian stable, and you'll notice that Debian has a newer bash:
[moonpie@osiris moonpiedumplings.github.io]$ podman run -it --rm debian
root@980ac170ddb4:/# bash --version
GNU bash, version 5.2.37(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
root@980ac170ddb4:/# exit
exit
[moonpie@osiris moonpiedumplings.github.io]$ podman run -it --rm ubuntu
Resolved "ubuntu" as an alias (/etc/containers/registries.conf.d/00-shortnames.conf)
Trying to pull docker.io/library/ubuntu:latest...
Getting image source signatures
Copying blob 817807f3c64e done |
Copying config f794f40ddf done |
Writing manifest to image destination
root@1486a1c38699:/# bash --version
GNU bash, version 5.2.21(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
This is Ubuntu 24, the current stable release. 25/questing, the rolling version does have newer/same package versions of debian. But people don't base distros off of the rolling version of ubuntu, only the stable releases.

The way forgejo actions works, is that it is not a universal thing for every repo. Each repo, can have it's own forgejo actions instance connected to it, running stuff.
The big benefit of that, is that you can make users bring their own actions servers, and not bother to deploy your own.