The default for flyway.table has been changed from schema_version to flyway_schema_history. And they have also provided automatic fallback to old default with a warning to avoid breaking existing installations using the old default.
It means from flyway 5, If you do not specify flyway.table property inside your configuration file, then flyway will look for the table flyway_schema_history in db, and if not found it will look for the table schema_version as a fallback and if the old table is found then will warn with the message that you are getting now. From flyway 6, this fallback mechanism will be removed. If you do not provide flyway.table property, it will look for flyway_schema_history in db, if not found it will not look for schema_version table even if you have any and will create a new table named flyway_schema_history to maintain functionality.
In flyway 6, your existing system will run fine if you set flyway.table=schema_version, you do not need to change table name in db. But if you do not set the property, then you must have to change the table name, otherwise flyway will not recognize existing schema_version table, will treat the system as a new one, will create flyway_schema_history table and will start executing scripts from start.
Hoping it will help.