What schema does this mean 'mysql - schema's default character set: utf8'?
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.