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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

I've also set a 3 day "bake period" which should prevent autoupgrading to a malicious version. That's one learning from the tj-actions/changed-files supply chain issue that I wanted to make sure to take into account.

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