Rename bucket to data set

This should work in conjunction with the change to the status check in this pull request:

https://github.com/alphagov/pp-puppet/pull/376/files

Only references to buckets which remain are in migration referencing the old buckets collection, code around dumping old configs from this collection and code in a task to load bucket config seeds which is invoked in a data migration.

I have not yet tackled references in pp-deployment, stagecraft (only a couple there), spotlight, pp-manual, libretto (old name?), limelight (still used?), performance-platform (?), backdropsend (?) or the collectors but I have surveyed these and this should work on its own. None of the references in these projects are massively complicated or have cross app dependencies on the word bucket and I have tested as much as possible locally so it would be good to try out this on preview before continuing.