Remove mention of "TiFlow" from "Best Practices for TiDB Security Configuration"

[kennytm]

Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

https://docs.pingcap.com/tidb/stable/best-practices-for-security-configuration/#protect-internal-ports

This section mentioned 4 ports associated with "TiFlow". However, there is never a unified "TiFlow" product released to public. We still refer to "TiCDC" and "DM" separately.

2. Describe your suggestion or addition.

Refer to those 4 ports by their actual component.

Component	Default port	Protocol
DM master	8261	HTTP
DM master	8291	HTTP
DM worker	8262	HTTP
TiCDC	8300	HTTP

3. Provide some reference materials (such as documents and websites) if you could.