Add missing core renderers and clarify deprecation for legacy_format_renderer

In MDL-71135, the course/classes/output/course_format/legacy_renderer.php was added because, since Moodle 4.0, renderer.php file was optional (although highly recommended) for course formats but from Moodle 4.0 onwards renderer is required to support the new course editor implementation.

This legacy class was created for backward compatibility, to avoid some errors with course formats (such as social) without this renderer.php file.

This issue has been created to add missing core renderers (i.e. social) and to clarify the deprecation process for this legacy_format_renderer class.