cli_changefeed_update report pd is an unsupported flag

[XuJianxu]

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.
   Use update to update changefeed
   "/cdc cli changefeed update "--pd=http://upstream-pd.testbed-jianxu-bggzq.svc:2379\" "--changefeed-id=basic-incremental-sync-1" "--no-confirm" "--config=/tmp/changefeed.toml""
2. What did you expect to see?
   no error happened
3. What did you see instead?
   "/cdc cli changefeed update "--pd=http://upstream-pd.testbed-jianxu-bggzq.svc:2379\" "--changefeed-id=basic-incremental-sync-1" "--no-confirm" "--config=/tmp/changefeed.toml""

{"stdout": "[2021/09/13 08:43:15.769 +00:00] [WARN] [cli_changefeed_update.go:195] ["unsupported flag, please report a bug"] [flagName=pd]
4. Versions of the cluster

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

    ```
    nightly
    ```

- TiCDC version (execute `cdc version`):

    ```
    nightly
    ```

[Rustin170506]

/assign

[Rustin170506]

We need to ignore these global parameters here.

If anyone would like to help fix it, I'd be happy to offer my help.

/unassign

[Rustin170506]

/assign

[Rustin170506]

Fixed by https://github.com/pingcap/ticdc/pull/2840.