x/build/cmd/gomote: notify gomote users of upcoming change to authentication

[cagedmantis]

In preparation for a change in the how gomote authentication will function, a notification should be added to gomote. This notification will alert users that a change will be happening in the near future. The preamble that is sent when a gomote instance is created seems like the appropriate place for this notification. Once gomote users can request credentials via the new path, the notification should be updated to instruct users on how to request credentials #48725

This is a component of the project to revamp the security model used by gomote #47521
@golang/release

[cagedmantis]

Deemed unnecessary. An announcement will be mailed out.