Add downstream release note about deprecated `MonitoredTables` snapshot metric

The MonitoredTables option in the Debezium connectors snapshot metrics was deprecated in the 1.5 release upstream. To comply with productization requirements to notify customers about deprecated functions at least one release in advance of their removal, we'll retain documentation about the option in the Q3 RHI release.

For the follow-on release, (2021-Q4 or 2022-Q1) add a release note to advise that the option is removed.