fix: Use authentication_policy to specify default auth plugin

`default_authentication_plugin` was removed starting in MySQL 8.4.0:

> The default_authentication_plugin system variable, deprecated in MySQL 8.0.27, is removed as of
> MySQL 8.4.0. Use authentication_policy instead.
>
> -- https://dev.mysql.com/doc/refman/8.4/en/mysql-nutshell.html

Signed-off-by: Siddharth Kannan <mail@siddharthkannan.in>

config/mycnf/mysql84.cnf

@@ -14,7 +14,7 @@ mysqlx = 0
 # 8.4 changes the default auth-plugin to caching_sha2_password and
 # disables mysql_native_password by default.
 mysql_native_password = ON
-default_authentication_plugin = mysql_native_password
+authentication_policy = 'mysql_native_password'
 
 # Semi-sync replication is required for automated unplanned failover
 # (when the primary goes away). Here we just load the plugin so it's