Flu vaccine lasts 6 months instead of forever

[wapcaplet]

Summary

SUMMARY: Bugfixes "Flu vaccine lasts 6 months instead of forever"

Purpose of change

To make the flu vaccine in game have a realistic effect length (6 months, rather than forever).

Describe the solution

Change the flu_vaccine function to use a 6-month duration, and not use a permanent effect.

Describe alternatives you've considered

The real flu vaccine doesn't guarantee immunity, but I'm not sure how to model that. Giving it a non-infinite duration seemed like a good compromise.

Testing

In-game, to ensure the duration and permanence did what I expected:

• Tested a 10-minute vaccine, non-permanent, checked that it wore off in 10 minutes
• Tested a 10-minute vaccine, permanent, checked that it did not wear off in 10 minutes

Additional context

The 30-day duration was added in #33243 but the "permanent" flag predates it. 30 days much too short for a realistic flu shot effect anyway:

The effects of the vaccine generally only last through one flu season,
which is about six months long.
https://www.verywellhealth.com/how-long-does-a-flu-shot-last-770538

How long does immunity from influenza vaccine last?
Protection from influenza vaccine is thought to persist for at least 6 months.
https://www.immunize.org/askexperts/experts_inf.asp

All right, how long does the flu shot last?
The short answer: six months. But the long answer is a little bit more complicated
https://www.health.com/condition/cold-flu-sinus/how-long-does-the-flu-shot-last