Bump govuk_content_models to 24.2.0

This adds support for the 'latest_change_note' and 'public_timestamp'
fields to the Artefact model.

A dependency change in 24.1.0 requires at least version 10.0.0 or above
of gds-sso, so the gem has been updated here too.
Include change information when indexing artefacts

This changes the RummageableArtefact model to include the 
'public_timestamp' and 'latest_change_note' fields when rendering an
artefact to be sent to Rummager.
Merge pull request #234 from alphagov/send-public-timestamp-and-change-note-to-rummager

Send public timestamp and change note to rummager