Replace mentions of the `ci-linux:production` image tag with the `ci-unified:latest`

[rcny]

Hello everyone!

I see that your project uses one of the paritytech CI images. Just FYI, the paritytech/ci-linux:production image has been deprecated in favor of the paritytech/ci-unified image. Please use the paritytech/ci-unified:latest image in your workflows, pipelines or docs, or use the specific tag (e.g. paritytech/ci-unified:bullseye-1.70.0) for granularity and versioning, but you will still need to check upstream for new releases.