[DNM] rename resolved_ts to watermark

[zhangjinpeng87]

Part of tikv/tikv#16708

• Rename resolved_ts to name watermark if it is a field. Rename proto filed name without change its id is compatible.
• Keep PD API rpc ReportMinResolvedTS(ReportMinResolvedTsRequest) returns (ReportMinResolvedTsResponse) {} for compatibility reason, at the same time add rpc ReportMinWatermark(ReportMinWatermarkRequest) returns (ReportMinWatermarkResponse) {} which will replace the former one in the future.

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