Transition from dependabot to renovate

[DanielVoogsgerd]

I tested this last week on my own fork, and while is can be tedious/fiddly at times to set up, and we won't nail it the first time for sure. I still think it is a huge upgrade coming from Dependabot, which right now is basically broken when it comes to cargo maintenance.

I have a config on my fork that would just fine for us, but it will throw a lot of PRs at first (~30 iirc), which I rather postpone until lib-refactor has been merged and the remaining major upgrades have been completed.