Drupal 12 compatibility: add ^12 to core_version_requirement and fix removed/changed APIs [#3629655]

Problem/Motivation

Drupal 12 is scheduled for GA in December 2026. The latest release of csv_serialization (4.0.1) does not declare ^12 in core_version_requirement, so Composer cannot install it on Drupal 12.

Proposed resolution

Add ^12 to core_version_requirement and update code that breaks on Drupal 12 (removed or changed core APIs). The change touches 1 file(s), about 2 changed line(s).

Related: #3596882: Automated Drupal 12 compatibility fixes for csv_serialization 4.x-dev The change is attached to this issue as a patch (generated against 4.0.1).

Remaining tasks

Review the merge request
Merge and tag a release that includes ^12

Needed by the YMCA Website Services (Open Y) distribution for Drupal 12.

Issue fork csv_serialization-3629655

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