Drop Drupal 9 support and require Drupal >= 10.3

Problem/Motivation

Drupal 9 reached end of life on 1 November 2023, but the module still declares core_version_requirement: ^9 || ^10 || ^11. Keeping it means carrying BC shims for a core version nobody should be running, and it blocks moving the handler plugin type to PHP attributes, which needs Drupal 10.2 or newer.

Proposed resolution

Set core_version_requirement: ^10.3 || ^11 and drop any code kept only for older core.

Remaining tasks

• Update conditional_fields.info.yml.
• Remove Drupal 9 and 10.0–10.2 era workarounds, and update the README if it mentions supported versions.
• Confirm the GitLab CI pipeline is still green, including the OPT_IN_TEST_PREVIOUS_MAJOR job.
• Follow-up, separate issue: convert the ConditionalFieldsHandler plugin type and the 15 handler plugins to PHP attributes.

API changes

Sites on Drupal 9, or on 10.0 to 10.2, cannot update past this release. To be called out in the release notes.