Drupal 11 upgrade path for hooks

Problem/Motivation

Drupal 11 switched from the hooks in the *.module file to \Drupal\Core\Hook\Attribute\Hook attribute.
In the new release, we are planning to make the module requirement drupal/core >= 10.3, so, at least we need to prepare the future hook class. So, in the future, when the module requirement will be drupal/core => 11.1 we can easily switch to the hook attribute and just remove the module file.

Proposed resolution

- first see a doc on how to make this migration correctly, to support D11 and D10 at the same time
https://www.drupal.org/node/3442349
- Make src/Hook/FormHooks and move the code of pdf_to_imagefield_form_alter there
- Make src/Hook/EntityHooks and move the code of pdf_to_imagefield_convert_pdf there
- in the module file use \Drupal::classResolver() to call the moved code from the expected classes
- Remember that the hook_entity_insert of the module must be handled after all other same hooks (according to pdf_to_imagefield_module_implements_alter). This means that the hook attribute must have the order parameter (Order::Last)