Create PR-bot to update op-geth versions (docker / go.mod)

[ezdac]

Currently the docker base container for building the l2 node (op-geth) in the local devnet has
to be manually updated.

For this, the correct sha256 digest from the image at blockchaintestsglobaltestnet/us-west1/dev-images/op-geth/ tagged with the most recent default branch commit has to be found.

Additionally the go dependency should reflect the same version of op-geth in the go.mod file's replace directive.

It would be good to update this regularly, especially as soon as we do more testing of Celo specific STF features in the optimsim e2e suite.
This could be achieved by a Github action that opens a pull-request with seding the go.mod and Docker files.