last_updated field missing from JournalEntry REST API serializer

[jeremystretch]

NetBox version

v3.2.5

Python version

3.8

Steps to Reproduce

1. Create one or more journal entries
2. Retrieve the /api/extras/journal-entries/ REST API endpoint

Expected Behavior

The last_updated field should be included.

Observed Behavior

The last_updated field is missing.