Steps to reproduce:

1. Export 4.0 schema as sql script (mysql 5.7)
2. Change character set to utf8mbr, collation utf8mb4_bin int the sql script
3. Create schema using the above sql script in a new database (mysql 8.0.26)
4. Export 4.0 data from old database (mysql 5.7)
5. Import 4.0 data into new database (mysql 8.0.26)
6. Start zabbix_server process
7. Zabbix server tries to upgrade schema and fails: [Z3005] query failed: [1071] Specified key was too long; max key length is 3072 bytes [create index items_1 on items (hostid,key_(1021))]

Result:
See log file...
Expected:
Zabbix server should be aware of the limitations on index keys on MySQL 8+ when using utf8mb4 character set (max key length is 3072/4=768)