merge develop into production

• Add flo to prod: ssh key + root access (!292 (merged))
• Add flo's ssh key to the staging grid root user authorized_keys (!291 (merged))
• Configure journald to discard logs before they're older than 30 days (!285 (merged))
• Fix docs code blocks (!289 (merged))
• nixpkgs update 2022-04-13 (!287 (merged))
• Add update-nixpkgs / diff-closures documentation (!288 (merged))