Enable new column backfiller by default and fix DB variable update

[tbenr]

This PR also make the backfiller the only one responsible of writing earliestAvailableCustodySlot db variable.
see #10318

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

---

Note

Medium Risk
Changes default behavior to enable the reworked column custody backfiller and adjusts how the custody cursor DB value is advanced, which can impact sync/backfill behavior and resource usage on real networks.

Overview
Enables the reworked data-column custody backfiller by default by switching the default for --Xp2p-reworked-sidecar-custody-sync-enabled (and the underlying P2PConfig default) from disabled to enabled.

Fixes DasCustodyBackfiller so when the stored earliestAvailableCustodySlot is at/behind the moving minCustodyPeriodSlot, it writes the cursor forward to minCustodyPeriodSlot instead of just exiting, and adds a unit test covering this cursor-alignment case. Acceptance tests are updated to stop explicitly enabling the reworked custody sync, and the changelog notes the backfiller being enabled.

^{Written by Cursor Bugbot for commit 8ba9e1f. This will update automatically on new commits. Configure here.}

[zilm13]

LGTM