AO3-6636 Update chapter cache key

[domenic]

This ensures that speculation rules prefetching is output, and cached, for all chapters.

This is an alternative to #4668.

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• Have you added tests for any changed functionality? Per discussion on the ticket, no tests are needed for this feature
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
• Do you have fewer than 5 pull requests already open? If not, please wait
  until they are reviewed and merged before creating new pull requests.

Issue

https://otwarchive.atlassian.net/browse/AO3-6636

Purpose

Ensure the introduction of speculation rules next chapter prefetching causes the chapter to be evicted from cache, but then cached going forward under the new v5 cache key.

Testing Instructions

General testing instructions for the feature are on the issue, but for this caching issue specifically, @sarken suggests testing on works which had previously been generated this change, maybe?

References

• This comment https://otwarchive.atlassian.net/browse/AO3-6636?focusedCommentId=360659
• Discussions with @sarken and @zz9pzza

Credit

Domenic Denicola, he/him