Remove SSLv3 support

[kubo39]

• SSLv3 is susceptible to downgrade attacks a.k.a. POODLE 5 years ago.
• rfc7568 says SSLv3 MUST NOT be used.
• Mozilla called [the end of SSL 3.0](SSLv3 MUST NOT be used) in 2014.
• couldflare: SSLv3 Support Disabled By Default in 2014.
• Google diabling SSLv3 in 2014.

I'm not sure why SSLv3 is still supported by vibe.d, but it's time we remove it, as it's not just obsolete, but insecure.

[Geod24]

Vibe.d does not implement SSL directly, instead relying on either OpenSSL or Botan (with OpenSSL the default).
AFAICS SSLv3 was disabled because OpenSSL disabled it: #1316

The only thing left to do, AFAICS, is

vibe.d/tls/vibe/stream/tls.d

Line 337 in 03b8589

enum TLSVersion {

[kubo39]

If I'm correct, this PR only says that SSLv3* methods were removed from OpenSSL on debian.
Although vibe.d disabled SSLv3 by default, SSLv3 connections can still be achieved by passing the SSL_OP_NO_SSLv3 options to OpenSSL.