Add alt text to existing Small Y demo media images

Follow-up to !85 (merged), which adds alt text to the Small Y demo media migration for new installs. This adds ws_small_y_update_91009 for existing sites: for media created by small_y_demo_media_image (looked up in migrate_map_small_y_demo_media_image) whose image alt is empty, it sets the alt from the demo migration rows. Alt text already set by site editors is kept. The rows are read from the small_y_demo_content YAML file because that module is uninstalled after the site install. Testing: on a site installed before !85 (merged), run drush updb; SELECT COUNT(*) FROM media__field_media_image WHERE field_media_image_alt <> ''; goes from 0 to 132, an alt edited by hand before the update stays unchanged.