set_lower_bounds script improvements

[mulkieran]

Currently, we can get into a situation where we can't update a packaged to its lowest bound automatically, because that package is also dependended on transitively by some package that continously updates its lowest required version. The example is: https://github.com/stratis-storage/libcryptsetup-rs/actions/runs/6003188610/job/16281095812#step:8:175, where we see that rustix constantly updates the lowest version of nix that it allows.

We would like to have some way to automatically push back some of these transitive dependencies. A general approach searches the dependencies in some kind of breadth-first order. For example, if pushing back direct dependencies does not succeed, then pushing back secondary dependencies should be tried.

An alternative is to specify per-package specific dependencies to push back.