use dependabot instead of CompatHelper

[Krastanov]

I recently ran an automated job that moved most JuliaGraphs repos away from CompatHelper onto using dependabot (dependabot has many advantages over CompatHelper, including that it never stops running, even after the repo has been inactive for a while).

Given the monorepo structure here, the script did not succeed to automatically make the changes.

Here are some example PRs making such changes: JuliaGraphs/Graphs.jl#479

I am creating this issue to track the potential switch to this newer better compatibility update tool.