mod_ssl: Disable SSL v3 (and v2) protocols by default.

Both the SSL v2 and v3 protocols have known problems, and the TLS v1.x
protocols are available to almost everyone.  One known exception is
Internet Explorer 6 on Microsoft Windows XP, but XP is out of support
anyway, so hopefully few people run that.  Users of mod_ssl can override
this, but defaulting to not use insecure protocols is the proper thing.