Fix deletion of expired ChangesetSpecs

Created by: mrnugget

The comments in the code explain what was wrong before:

• We tried to delete CampaignSpecs before the ChangesetSpecs, which would lead to a foreign key constraint violation.
• We expired ChangesetSpecs that were still attached to a Changeset, either as the PreviousSpec or the CurrentSpec.

This commit fixes both issues and fixes #12763.