In the backend of JEvents in the control panel this message should give you the option to update the database collations. If you click this it will change your text based fields to a structure that allows more complex data to be stored such as emojis etc.

If you continue to see this message after applying the update can you let me know and I#ll take a look

Thanks geraint! After updating, I did still see the message below. However, after navigating away, then back, the message disappeared for each site. So I think it should be good to go.