OK, good - the database newmattermost has been created with the correct collation.
Please make sure that the database which is being used by the updated mattermost installation on the new server uses a database, which collation is set to utf8mb4_0900_ai_ci - you know how to check that now 
Not sure if you imported it to newmattermost now or to any other database name, but whatever database it is, it should have the correct collation before you try to run the upgrade again.