Bump version to 24.0.0

There are multiple changes being released:

* Major clean-up which replaced `WorkflowActor` with
`ActionProcessors`. this is a breaking change, shouldn't
break any existing functionality, but may break re-opened
classes, and tests relying on deleted workflow helper
methods.
* Added Edition fields `major_change` and `change_note`
* Improved the audit log for scheduled publishing to
show scheduled time, by storing it in the `action`.
Merge pull request #244 from alphagov/bump_version

Bump version to 24.0.0