Mention Workhorse mainainers in code review doc

There is a table in doc/development/code_review.md that lists who should review what kind of changes. There was no mention in that table that Workhorse changes should be reviewed by a Workhorse maintainer. This MR adds Workhorse to the table.

This is motivated by a recent MR !92604 (merged) that got merged without any Workhorse review, even though it made Workhorse changes.