Rewrite Security/TLS page to not be historical

The whole last section on TLS in [security doc](https://etcd.io/docs/v3.6/op-guide/security/) for etcd is a historical journal of releases going back to 3.1. This is not useful. It should get rewritten to document what features and configurations etcd supports *now*, and should not have information about how it was supported differently in the past.

Start with the linked Security/TLS page and read the entire last section, noting which material describes historical releases rather than current behavior. Rewrite that section around the TLS features and configurations etcd supports now, removing past-version comparisons; done means the page documents current support without the release journal.

Written by the indexing model from the issue text.