Renamed the #prefix template variable to #note

As #prefix is actually kind of a “reserved” name in the theme layer, which will automatically be prepended to the output when rendering, its use of it in our search_api_autocomplete_suggestion theme function/template was ill-advised. The variable was therefore changed to #note, custom overrides of that template (search-api-autocomplete-suggestion.html.twig) will have to be adapted accordingly.