Make PostgresModelDataMigrator work for scheduled rebuilds of a table

[Photonios]

• Don't clone the foreign key constraint of a field with db_constraint=False. clone_model_constraints_and_indexes_to_schema looked up a constraint that does not exist and the unpack raised a ValueError, so the migrator could not migrate such a model at all

• Delete the work and backup schemas when a phase fails. They were left behind, half-filled, on every failed migration

• Add keep_backup_schema, default on. Turned off, the backup schema is deleted after a successful migration, so a migration that runs every day does not leave a copy of the table behind on every run

• Move the CI images from bullseye to bookworm. The bullseye security packages are gone from the Debian mirrors, so installing packages failed every job

• Fix release.yml not parsing, which failed a run on every push and would have failed a release. Its version check also used [[, which the container's default sh does not have