[llvm] [Github] Add renovate configuration for Github Actions (PR #159899)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Alright, I think this is ready for review. I've set this up to only look at Github Actions and create a PR once a week with version updates. The PR will assign to me for now while I iron out config changes, but we can update later if more people want to get pinged.

https://github.com/llvm/llvm-project/pull/159899