Unable to restore v7.9.2 backup to v12.4.0


I’m trying to restore the backup of an old GitLab v7.9.2 (created by gitlab-rake gitlab:backup:create) to our new Debian 9 machine running GitLab v12.4.0 (using gitlab-backup restore BACKUP=timestamp`). The restore errs out this way:

Unpacking backup … done
GitLab version mismatch:
Your current GitLab version (12.4.0) differs from the GitLab version in the backup!
Please switch to the following version and try again:
version: 7.9.2

Hint: git checkout v7.9.2

How can I go about migrating from v7.9.2 to v12.4.0?