I ran this query on my keyserver's production MariaDB instance to see what codeVersions were using Comm.
It's possible that users are logged out on older clients, and might start using those older clients again... so this isn't a guarantee that older clients don't exist.
But for those users, throwing new ServerError('client_version_unsupported') here will cause them to display an Alert prompting the user to upgrade their client. So I don't think it's a big concern.
The main impetus for this change is that it will allow us to improve performance for eg. thread joins by addressing this, which will potentially solve ENG-108, and other recent complaints (ENG-4376, ENG-4396).
This diff just bump the minimum required codeVersion, but following diffs simplify code where possible to reflect the new minimum required codeVersion.