Update CockroachDB docs to advise against sslmode=require

[rafiss]

Rafi Shamim (rafiss) commented:

In this part of the CockroachCloud docs we say:

You can also use the require SSL mode, although we do not recommend using it since it can make the cluster susceptible to MITM and impersonation attacks. For more information, see the “Protection Provided in Different Modes” section in PostgreSQL’s SSL Support document.

This should also documented in this part of the CockroachDB docs.

@lnhsingh and @piyush-singh also raised the point that perhaps we should not be linking to the PostgreSQL documentation at all, but as a starting point I think we'd still want to include this information in CockroachDB docs in some form.

Jira Issue: DOC-963