Write cutover for work_item_positions (rebalancing path)

What does this MR do and why?

First slice of the write cutover for work item relative positioning.

• Adds the feature flag write_relative_positions_to_work_item_positions (gitlab_com_derisk, default off) and the Issue.write_relative_positions_to_work_item_positions? gate.
• Redirects the rebalancing service's position UPDATE (Issues::RelativePositionRebalancingService) to work_item_positions when the flag is on.
• The flag-off path is byte-identical to today, so this change is dormant until the flag is enabled.

This is part of the write cutover (#612911) under epic &23222, which makes work_item_positions the source of truth for position writes instead of issues.relative_position. The read side already moved in !248883 (merged). Later slices (core move path, move_nulls, update_relative_siblings, dropping the sync trigger) will follow as separate MRs — do not enable the flag until those land, or issues and work_item_positions will diverge.

No changelog entry (behind a default-off flag). Added a spec for the flag-on path (rebalance writes to work_item_positions, leaves issues.relative_position untouched); existing specs already cover the flag-off path.

Query Plan

References

Part of #612911, epic &23222, rollout #627322. Follows read-side MR !248883 (merged).

How to set up and validate locally

1. Enable both flags in the rails console:

   Feature.enable(:read_relative_positions_from_work_item_positions)
   Feature.enable(:write_relative_positions_to_work_item_positions)

2. Trigger a rebalance, e.g.:

   Issues::RelativePositionRebalancingService.new(namespace).execute

   on a namespace with issues.
3. Confirm work_item_positions.relative_position is updated (spread out) and issues.relative_position is unchanged.

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