cdc cli 5.1.0 + cdc server 4.0.14, create changfeed default sort-engine is memory

[Tammyxia]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do? If possible, provide a recipe for reproducing the error.

• Create changefeed with cli 5.1.0: $tiup cdc cli changefeed create --pd=http://172.16.xxx:2379 --sink-uri='mysql://root:@172.16.xxx:4000/' --changefeed-id="replication-task" cdc-2021-07-22T16-11-29.795.log:[2021/07/22 14:43:00.651 +08:00] [INFO] [owner.go:301] ["Find new changefeed"] [info="{"sink-uri":"*","opts":{},"create-time":"2021-07-22T14:43:00.56923044+08:00","start-ts":426491558124847105,"target-ts":0,"admin-job-type":0,"sort-eng
  ine":"memory","sort-dir":"","config":{"case-sensitive":true,"enable-old-value":false,"force-replicate":false,"check-gc-safe-point":true,"filter":{"rules":["."],"ignore-txn-start-ts":null},"mounter":{"worker-num":16},"sink":{"dispatchers":null,"protoc
  ol":"default"},"cyclic-replication":{"enable":false,"replica-id":0,"filter-replica-ids":null,"id-buckets":0,"sync-ddl":false},"scheduler":{"type":"table-number","polling-time":-1}},"state":"normal","history":null,"error":null,"sync-point-enabled":false,
  "sync-point-interval":600000000000,"creator-version":"v5.1.0"}"] [changefeed=blackhole-task-1] ["checkpoint ts"=426491558124847105]

2. What did you expect to see?

• sort-engine is unified by default.

3. What did you see instead?

• sort-engine is memory by default
• While cli 4.0.14 create changefeed sort-engine is unified by default, so it's compatibility issue between cdc cli 5.1.0 and cdc server 4.0.14.

4. Versions of the cluster

   • Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

     4.0.14
     

   • TiCDC version (execute cdc version):

     4.0.14