MassInstallAction: Install the Downgrade workflow on this repository

[Krastanov]

This pull request sets up the Downgrade workflow on this repository. cc: @Krastanov

[Krastanov]

The value of having this in CI is to ensure you are not inadvertently depending on features available only in recent versions of your dependencies. It also helps against accidentally having your users resolve to an antiquated version of dependencies (that is not really checked in CI). For more info, check this discussion: https://discourse.julialang.org/t/psa-add-downgrade-ci-to-better-check-version-compatibility/110063 (I will add a newer version of this action that solves some resolver issues, available since Julia con and already used in the SciML ecosystem)

It has already detected broken lower compat bounds in other repos in JuliaGraphs

The reason the tests are failing is that many of the actions used in the tests are deprecated. This can be automatically fixed by dependabot, which can be added by simply adding a file like this one to the repository: https://github.com/JuliaGraphs/Graphs.jl/blob/master/.github/dependabot.yml

I am submitting PRs like this to most JuliaGraphs repos. I am happy to help with further work on this repo with Downgrade and dependabot, but for now I am leaving this to the current active maintainers. Just let me know if there is anything on which there are any questions.