Issue #3576977: D12 compatibility - hook class migration and core_version_requirement update

Fixes #3576977

This MR prepares dropzonejs for Drupal 12 compatibility:

• Adds src/Hook/DropzoneJsHooks.php with all hook implementations moved to a class using #[Hook] attributes
• Updates dropzonejs.module to delegate from procedural #[LegacyHook] wrappers to the new hook class
• Registers Drupal\dropzonejs\Hook\DropzoneJsHooks in dropzonejs.services.yml with autowire: true
• Updates core_version_requirement in both dropzonejs.info.yml and dropzonejs_eb_widget.info.yml to ^10.3 || ^11 || ^12 (drops D9 support, adds D12)

This builds on the existing MR !29 which handles the deprecated API replacements (FormElement, FileSystemInterface::EXISTS_RENAME, etc.) — that MR should be merged first or this work should be rebased on top of it.