What schema does this mean 'mysql - schema's default character set: utf8'? While upgrading mysql from 5.6 -&gt; 5.7 -&gt; 8.0.23 in step 5.7 -&gt; 8.0.23 I got a warning:
The following objects use the utf8mb3 character set. It is recommended to convert them to use utf8mb4 instead, for improved Unicode support.
More Information:
https://dev.mysql.com/doc/refman/8.0/en/charset-unicode-utf8mb3.html

common_schema - schema's default character set: utf8
mysql - schema's default character set: utf8
common_schema._global_qs_functions.function_arguments - column's default character set: utf8
common_schema._global_qs_variables.value_snapshot - column's default character set: utf8
common_schema._global_script_report_data.info - column's default character set: utf8
...

So what means 'mysql - schema's default character set: utf8'? how can I change it to utf8mb4?
common_schema is a tool that someone added to your server?  Sounds like it has not been updated to utf8mb4.
CHARACTER SET utf8 (aka utf8mb3) is old; utf8mb4 is new.  The latter can handle Emoji and more of the Chinese character set, plus some less common languages.
If you don't know anything about common_schema, ignore the warning.
In your tables, consider using utf8mb4.  This is the default in 8.0; you would have to take deliberate action not to use it.