Reduce WebHookLog retention from 14 to 7 days

What does this MR do and why?

Reduce WebHookLog retention from 14 to 7 days

Every read path for webhook logs (GraphQL, REST, and the project settings UI) already caps results to WebHookLog::MAX_RECENT_DAYS (7), so logs older than 7 days were already unreachable through any list view. Aligning retain_for with that existing cap removes storage that was never exposed to users, other than direct links to a specific log entry.

This detaches and drops partitions for days 8-14 on the next Database::PartitionManagementWorker run. Detached partitions remain physically recoverable for 2 days (retain_detached_partitions_for:) before Database::DropDetachedPartitionsWorker permanently drops them.

Changelog: changed

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/604668

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