Support SHOW REPLICA STATUS for MySQL 8.0.22+

[psujit775]

Summary

• MySQL 8.4.0+ deprecated SHOW SLAVE STATUS, causing syntax errors when freno queries replication lag
• Implements a fallback: try SHOW REPLICA STATUS first, fall back to SHOW SLAVE STATUS on MySQL error 1064 (syntax error) for older versions
• Updates column names (Replica_IO_Running, Replica_SQL_Running, Seconds_Behind_Source) for the new syntax
• Updates documentation to reflect dual-syntax support

Backward compatibility

• MySQL >= 8.0.22: uses SHOW REPLICA STATUS directly
• MySQL < 8.0.22: falls back to SHOW SLAVE STATUS transparently
• No configuration changes required

Test plan

• script/test passes