Github Actions workflow to automatically sync critical forks (#5259)

This Workflow is not supposed to run in the paritytech/polkadot-sdk
repo. This Workflow is supposed to run only in the forks of the repo, in
`paritytech-release/polkadot-sdk` specifically, to automatically
maintain the critical fork synced with the upstream. This Workflow
should be always disabled in the paritytech/polkadot-sdk repo.