Clarify post-migration instructions for preserving SSH access on all Admin Workstations

[eloquence]

In all migration docs, we should clarify the procedure for preserving SSH access from additional Admin Workstations beyond the one that is used to perform the migration.

Depending on resolution for #174, this may involve copying the SSH keys; it certainly involves copying the Monitor Server's new onion address and v3 keys (its original config is not preserved).

The Focal Migration docs currently point to https://docs.securedrop.org/en/stable/v3_services.html#update-tails-v3; however, this is written from the point of view of ensuring v3 access after v2 is disabled, and it's not clear that it's a required step even for v3-only instances.

User Story

As an admin, I don't want to be caught by surprise after a migration that I can access app but not mon from one of my Admin Workstations, so that I can perform maintenance tasks without issues.