[lua] Quest Cleanup Promotion Superior Private

[slashtangent]

I affirm:

• I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• I have read and understood the Contributing Guide and the Code of Conduct.
• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Cleans up the quest to be retail accurate.
Adds missing event and message.

Steps to test these changes

1. !completequest 6 90
2. Set charvar AssaultPromotion to 25 or higher.
3. Talk to Naja_Salaheem to flag quest
4. Talk to Naja to see unique event.
5. Hunt down a Warhorse_Hoofprint and get a DARK_RIDER_HOOFPRINT or add it !addkeyitem DARK_RIDER_HOOFPRINT
6. Talk to Naja_Salaheem
7. Quest Complete

Captures

https://youtu.be/ZMHQySGdzgk
https://drive.google.com/file/d/1A1XpZVXtQhLQRfeWzcLzU8fO7JM8v_54/view?usp=sharing