Move hook_requirements() to the OOP requirements hooks for Drupal 11.3

Part of #3596034 (closed).

Problem/motivation

On Drupal 11.3 and later every request logs c2pa_sign_requirements without a #[LegacyRequirementsHook] attribute is deprecated in drupal:11.3.0 and removed in drupal:13.0.0. See https://www.drupal.org/node/3549685.

Proposed resolution

Follow the core pattern: implement hook_runtime_requirements() and hook_update_requirements() as OOP hooks that reuse the current checks, and mark c2pa_sign_requirements() with #[LegacyRequirementsHook]. Drupal 10 and 11 before 11.3 keep running the procedural function, 11.3 and later run only the OOP hooks.

AI Compliance

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