Migration docs should specify what to do with old id_rsa files

[eloquence]

The migration docs at https://docs.securedrop.org/en/stable/backup_and_restore.html#migrate-v3 (and again for the v2 path) currently specify what to do with the ~/.ssh/config and ~/.ssh/known_hosts files, but not what to do with id_rsa and id_rsa.pub. Since those files will be overwritten if you follow the reinstall procedure to the letter, we should make the desired behavior explicit.

[eloquence]

Discussed with @emkll @zenmonkeykstop and @rocodes, we'll recommend doing an SSH key rotation as part of the Ubuntu 20.04 migration.