Replace secrets with the new ones (#1564)

In the monorepo, secrets used in the various previous repos have been
renamed into:
- `CUMULUS_DOCKERHUB_USERNAME`
- `CUMULUS_DOCKERHUB_TOKEN`
- `POLKADOT_DOCKERHUB_USERNAME`
- `POLKADOT_DOCKERHUB_TOKEN`

This PR makes those changes and remove one of the GHW that has now been
updated for the monorepo.