Issue #3586114: Fix UserIdsTest flakiness on 4.x (notify threshold on 1-year boundary)

Frank Mablyrequested to merge
issue/purge_users-3586114:3586114-useridstest-flaky-on into 4.x

Moves the pre-notification threshold in the all_purge_conditions dataset of UserIdsTest off the 1-year boundary that coincides with strtotime('-1 year') seeded users.

Changes user_before_notification_value from 18 to 12 months, so the notify threshold becomes 30 - 12 = 18 months — safely between the seeded 1-year and 2-year marks. The expected notify list is unchanged.

Mirrors what 5.x already does (introduced via policy refactor in #3427680) and what #3411228 already did for the mixed dataset on 4.x.

Refs #3586114.

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