Amend supported OS deprecation notice

https://couchbase.slack.com/archives/DQK2LD55X/p1589551591008900

Remove the "Support for deprecated Operated Systems will be removed four releases following the deprecation notice."  https://docs.couchbase.com/sync-gateway/2.7/supported-os.html#deprecated-versions and align it with what we added in CBL "Deprecated versions will be removed in a future release and we recommend that you plan to migrate away from the deprecated OS versions."