Unknown initial character set index ‘255’ received from server

Some further investigation showed that the issue was exactly in changes which were done in MySQL v.8.0:

Character Set Support

Important Change: The default character set has
changed from latin1 to utf8mb4. These system variables are affected:

The default value of the character_set_server and
character_set_database system variables has changed from latin1 to
utf8mb4.

The default value of the collation_server and
collation_database system variables has changed from latin1_swedish_ci
to utf8mb4_0900_ai_ci.

All these change were already processed in new version of mysql-connector-java and there is no need to configure your MySQL. So change from 5.1.6 to 5.1.44 fix the issue:

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>5.1.44</version>
</dependency>