Make sure alter_cell_sequences_range rake is not celled from the legacy cell

Rake to update cell specific tables sequence li... (!168941 - merged) added the rake gitlab:db:alter_cell_sequences_range and it will be called from gitlab:db:configure (!170613 (merged)).

Since only new instances (includes cell provisioning) use gitlab:db:configure rake, the legacy cell (current Gitlab.com SaaS) will not alter the sequences. But it's more safe to add a check in the alter_cell_sequences_range to run only from new cells.

The name legacy has been decided in gitlab-com/content-sites/handbook!8287 (merged) (reference: slack internal).