Update dependencies in this repo, using Scala Steward / Renovate

[MPV]

For consumers of this action, running the Scala code in this action, I think it would make sense to keep its dependencies up-to-date.

I was thinking of either activating Renovate (albeit not-amazing Scala support).

Or maybe even more suitable, Scala Steward:

It seems it's possible to add this repo into this list, and benefit from automated upgrade PRs:

• https://github.com/VirtusLab/scala-steward-repos/blob/main/repos-github.md

It seems it's the new place for open source usage of Scala Steward, as per:

• https://contributors.scala-lang.org/t/can-scala-maintain-a-public-scala-steward-instance/5754/10

[MPV]

Ah, I see now that you've added it in your own Scala Steward instance: 🎉

• scalacenter/steward@7a06ebf