Doc: Use consistent messaging in off boarding doc

[synedra]

This doc: https://fusionauth.io/docs/lifecycle/migrate-users/offboard uses terms that we don't use elsewhere.

We don't call the two types of migrations static and dynamic or online and offline.

We should update the doc to use bulk and slow.

[mooreds]

I'd also be clear that the scheme of the offboarding doc is tied to a specific version and that someone making queries directly against the database will have to download our schema (https://fusionauth.io/direct-download) and reverse engineer it for their version.