In WT-13234, checkpoint cleanup can read pages from disk to remove their obsolete time window information.

This logic is done by checking if the time window is globally visible by comparing WT_MAX(newest_start_durable_ts, newest_stop_durable_ts) with the pinned ts.

Instead, we should compare the oldest_start_ts with the pinned ts to better identify pages with obsolete time window information.

However, oldest_start_ts cannot be currently used as it's not updated correctly.