entity.menu_position_rule.add_form route should use _entity_create_access requirement

Problem/Motivation

The entity.menu_position_rule.add_form route requires a permission explicitly.
It is a better practice to use the _entity_create_access requirement, this way access can be altered with hook_entity_create_access.

Steps to reproduce

Add this hook:

/**
 * Implements hook_entity_create_access().
 */
function test_entity_create_access() {
  return AccessResult::forbidden();
}

You can still create menu position rules.

Proposed resolution

Replace the _permission requirement with _entity_create_access.

Remaining tasks

User interface changes

API changes

Data model changes