Fix small issues in juniper-advanced example

[ethagnawl]

This PR addresses two issues I ran into while running the "juniper-advanced" example:

• Encoding error in mysql-schema.sql
  • I'm admittedly not sure how broadly useful this is, as I don't regularly use mysql but I had to replace instances of utf8mb with utf8 and utf8mb4_0900_ai_ci with utf8_general_ci in order to import the DB schema. I'm running: mysql Ver 15.1 Distrib 10.5.21-MariaDB, for debian-linux-gnu (x86_64) using EditLine wrapper
• Fix schema descriptions in root.rs which were referring to the wrong table (user => product)

[robjtede]

I had to replace instances of utf8mb with utf8

hmm this is definitely not right, utf8 is legacy and not really correct

[robjtede]

I'm guessing the newer utf8mb4_0900_ai_ci collation is not supported in your version of mysql. I've changed the charset back to utf8mb4 and updated the collate to utf8mb4_unicode_ci to support a larger range of mysql versions. Let me know if that combo works for you.