The UTF8MB3 character set is deprecated in 8.0

Migrate sys schema to UTF8MB4.

In particular:
- change UTF8 (meaning UTF8MB3) character set to UTF8MB4
- change utf8_bin collations to utf8mb4_bin
- change utf8_general_ci collation to the default (utf8_0900_ai_ci)

Patch for file scripts/mysql_sys_schema.sql
generated from upstream sys.

Adjust all test cases accordingly.