Remove secrest that are no secrets

[chevdor]

We use:

• DOCKERHUB_USERNAME: definitely no secret
• DOCKERHUB_TOKEN: that's a secret

We need to replace DOCKERHUB_USERNAME by paritytechcumulus and delete the DOCKERHUB_USERNAME secret.
I would take the chance during this change to rename DOCKERHUB_TOKEN to a more specific DOCKERHUB_PARITYTECHCUMULUS_TOKEN

cc @wabkebab

[chevdor]

See also paritytech/polkadot-sdk#588