FOSS broken because Gitlab::BackgroundMigration::UpdateVulnerabilityOccurrencesLocation::Occurrence is not defined

Job #1774415648 failed for 11c2b8ef:

# --- Caused by: ---
# NameError:
#   uninitialized constant #<Class:0x00007ffac7c58130>::Occurrence
#   ./db/post_migrate/20211102114802_update_vulnerability_occurrences_location.rb:11:in `up'