Disallow a request for timeline ancestor detach if either the to be
detached timeline, or any of the to be reparented timelines are
offloaded or archived.
In theory we could support timelines that are archived but not
offloaded, but archived timelines are at the risk of being offloaded, so
we treat them like offloaded timelines. As for offloaded timelines, any
code to "support" them would amount to unoffloading them, at which point
we can just demand to have the timelines be unarchived.
Part of #8088