TODO: "You need to upgrade" message

[jagerman]

On flag day we need SOGS to start returning a fake (i.e. signed by the server key converted into a Session ID) message saying something along the lines of:

"This server requires an upgraded version of Session."

when using the legacy message retrieval endpoints so that existing Session clients don't just silently fail (since they'll refuse messages posted with blinded keys, which starts on flag day).