Update SGW default minimum TLS to 1.2

Sync Gateway is defaulting to set the minimum TLS to 1.0.  [
https://github.com/couchbase/sync_gateway/blob/master/rest/config.go#L45]

https://github.com/couchbase/sync_gateway/blob/master/rest/config.go#L296

TLS 1.0 and 1.1 is no longer considered secure and has been deprecated by all Browsers and the majority of other software. 

We should set the minimum default TLS to 1.2 and work towards deprecation of TLS 1.0 and 1.1.    Note that in Couchbase Server 7.0, the default minimum TLS is 1.2, tracked as MB-41794 .