Background

The resolved_ts is used in TiKV， TiCDC, BR, TiDB, and many documentations like https://docs.pingcap.com/tidb/stable/ticdc-architecture. The term resolved_ts is hard to understand both on engineers' perspective and users' perspective.

In order to make the semantic more clearly, make the document more easy for our users, we should use a better alternative name for resolved_ts. The resolved_ts essentially is a watermark which is used by CDC, PiTR, flashback and EBS snapshot backup, etc.

Task Breakdowns

• kvproto [DNM] rename resolved_ts to watermark pingcap/kvproto#1237
• TiKV [DNM] rename resolved_ts to watermark #16727
• PD [DNM] rename resolved-ts to watermark pd#8012
• TiCDC [DNM] rename ResolvedTS to Watermark pingcap/tiflow#10872
• BR & TiDB [DNM] rename ResolvedTS to Watermark pingcap/tidb#52297
• Document

Timeline plan: merge after 8.1 LTS released.