httpd: Use highest TLS versions server supports

This is exactly the same change I made back in #9.  However, it needs to be made once again because #16 went back to whitelisting versions, without any explanation of why this is better.  I can, however, come up with several reasons why a blacklist is preferable:

1. The config will be suitable for versions of Apache/OpenSSL that don't support TLS 1.1 and 1.2.
2. Newer versions of TLS will be automatically enabled when supported.  There is, practically speaking, *no* chance those will decrease, rather than increase, security.