TUF repository deletion

[iliana]

We have endpoints for reading (system_update_get_repository) and creating (system_update_put_repository) TUF repositories in Nexus, but we need an endpoint for deleting one.

This is not as simple as removing the records from the database; we should prevent deleting repositories if Nexus might need the artifacts in order to manage the system. This might be as simple as ensuring we keep the "current" system version, the system version we want to be on by policy, and any versions in between.

[plotnick]

We should probably not even remove records from the database, as #7518 introduces a dependent target_release table. We can certainly add a time_deleted column, though.

[iliana]

Automatic TUF repo pruning was implemented in #9107 and #9109. Follow-up tasks:

• reuploading a pruned TUF repo should mark it unpruned #9138
• want ability to disable TUF repo pruner at runtime #9139
• always hand over uploaded TUF repos to the artifact replication task #9140
• drop generation_added column from tuf_artifact table #9141
• evaluate update-related datastore functions for check_complex_operations_allowed #9142