Callback before_footer in theme_snap component should be migrated to new hook callback for core\hook\output\before_footer_html_generation

• line 7470 of /lib/moodlelib.php: call to debugging()
• line 7534 of /lib/moodlelib.php: call to {closure}()
• line 71 of /lib/classes/hook/output/before_footer_html_generation.php: call to get_plugins_with_function()
• line 1505 of /lib/outputrenderers.php: call to core\hook\output\before_footer_html_generation->process_legacy_callbacks()
• line 142 of /index.php: call to core_renderer->footer()