Comment 3 for bug 1971565

From a charm's perspective, I believe we should be setting [DEFAULT].unknown_config_option to "warning", so (possible) future changes in this area won't break a running cluster.

From the docs https://dev.mysql.com/doc/mysql-router/8.0/en/mysql-router-conf-options.html#option_mysqlrouter_unknown_config_option :
"""
A warning is default behavior, and bootstrapping defines it as error in the generated configuration file. MySQL Router versions before 8.0.29 ignore unknown configuration options. A warning logs a warning message but does not halt, whereas an error means Router fails to initialize and exits.
"""