Drupal 9 Compatibility

Problem/Motivation

Module must be Drupal 9 compatible due to End-of-Life for Drupal 8, November 2021.

Proposed resolution

Replace core: 8, declare Drupal Core version requirement in be_ixf_drupal.info.yml
`core_version_requirement: ^8.8 || ^9`

Remove autoload because D9 does not need it unless the module provides a composer.json for module dependencies.