🐛 Migrate before_footer callback to Moodle hook

[DamienVauchel]

📋 Summary

• The legacy before_footer plugin callback triggers a deprecation notice on Moodle 4.4+ (core\hook\output\before_footer_html_generation->process_legacy_callbacks).
• Register the hook in db/hooks.php so the notice is suppressed and the callback runs through the new hook API.
• Keep mod_edflex_before_footer() (now returning its HTML) as a fallback for Moodle < 4.4.

🧪 Manual checks

• On a Moodle 4.5 site, open a SCORM activity with an edflex mapping: the edflex block still renders and no "should be migrated to new hook callback" debug notice appears.