copy_state: use a mediumblob instead of a smaller varbinary for lastpk (#18852)

Signed-off-by: Nick Van Wiggeren <[email protected]>

Author: nickvanw

go/vt/sidecardb/schema/vreplication/copy_state.sql

@@ -19,7 +19,7 @@ CREATE TABLE IF NOT EXISTS copy_state
     `id` bigint unsigned NOT NULL AUTO_INCREMENT,
     `vrepl_id`   int            NOT NULL,
     `table_name` varbinary(128) NOT NULL,
-    `lastpk`     varbinary(2000) DEFAULT NULL,
+    `lastpk`     mediumblob     DEFAULT NULL,
     PRIMARY KEY (`id`),
     KEY `vrepl_id` (`vrepl_id`,`table_name`)
 ) ENGINE = InnoDB CHARSET = utf8mb4