backup: dropped restore targets are unavailable in revision history backup

Suppose a revision history incremental backup captures data between t1 to t4. If a user drops a table or database at t3, they should be able to restore that target AOST t2, but currently they cannot, because we conduct target resolution for a given backup with the backup's end time.
https://github.com/msbutler/cockroach/blob/butler-database-drop-expanded-db/pkg/backup/backup_planning.go#L565

I don't feel the need to fix this urgently, but as we think about how to change descriptor handling for backup/restore with the 1 million descriptor project, we should keep this paper cut in mind.

Jira issue: CRDB-56085

Start with pkg/backup/backup_planning.go around line 565 and trace how target resolution uses the backup end time. Completion should allow a table or database dropped at t3 to be restored at AOST t2 from a revision-history incremental backup covering t1-t4; consider the descriptor-handling implications before proposing a change.

Written by the indexing model from the issue text.