Add a workflow to promote green commit to stable branch (#1697)

Summary:
This is ready now after https://github.com/pytorch/test-infra/pull/4905 lands.  I reuse the same convention from PyTorch here calling the stable branch `viable/strict`.

* I have already create the branch https://github.com/pytorch/executorch/tree/viable/strict as a protected one.  Only repo admin or the update bot can write into it.
* The update job is scheduled to run every 30m (copied from PyTorch)
* A commit can be promoted to viable/strict if all its pull and lint jobs pass.  The condition here can be extended to include trunk jobs too, but we need to get ET trunk into a healthy state before that.
  * My plan is to try to get lint and pull to a good state first, land this change, then fix trunk jobs, and add trunk signals too.

### Testing

https://github.com/pytorch/executorch/actions/runs/7648614817/job/20841686469?pr=1697#step:2:1270, no green commits was found because there are broken lint and pull jobs on main https://hud.pytorch.org/hud/pytorch/executorch/main

Pull Request resolved: https://github.com/pytorch/executorch/pull/1697

Reviewed By: guangy10

Differential Revision: D53072054

Pulled By: huydhn

fbshipit-source-id: 31dd1aec3a5c77ae4ed3639e63c4c6620340773e