CI/DEV: switch from dependabot to renovate

[lucascolley]

Not high priority IMO, but we can consider following https://pixi.sh/latest/integration/ci/updates_github_actions/

[lucascolley]

alternatively, it looks like renovate can now do this automatically: renovatebot/renovate#34646

[lucascolley]

@kgryte @rgommers I have requested access for the renovate bot on this repo, I think it needs Owner approval, whenever you have time, thanks!

[rgommers]

Done. That triggered gh-198. There's what I see after logging in (had to be done in order to authorize the app, grumble):