[Ruby SDK] - Add Legacy Durability documentation

Add mention of Legacy Durability support in Ruby documentation, something similar to this in Java:
https://docs.couchbase.com/java-sdk/current/howtos/kv-operations.html#durability

If a version of Couchbase Server lower than 6.5 is being used then the application can fall-back to ‘client verified’ durability. Here the SDK will do a simple poll of the replicas and only return once the requested durability level is achieved. This can be achieved like this:

We'll probably need to also add the section below to the Ruby "Failure Considerations" concept page:
https://docs.couchbase.com/java-sdk/current/concept-docs/durability-replication-failure-considerations.html#older-server-versions 

Older Server Versions

If a version of Couchbase Server lower than 6.5 is being used then the application can fall-back to 'client verified' durability. Here the SDK will do a simple poll of the replicas and only return once the requested durability level is achieved.
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