Fix error re-running some migrations if they get interrupted at the wrong moment (#17989)

lib/mastodon/migration_helpers.rb

@@ -813,6 +813,9 @@ module Mastodon
     def update_index(table_name, index_name, columns, **index_options)
       if index_name_exists?(table_name, "#{index_name}_new") && index_name_exists?(table_name, index_name)
         remove_index table_name, "#{index_name}_new"
+      elsif index_name_exists?(table_name, "#{index_name}_new")
+        # Very unlikely case where the script has been interrupted during/after removal but before renaming
+        rename_index table_name, "#{index_name}_new", index_name
       end
 
       begin