Change default TLS version of SSLContextConfig to TLS 1.2+
Change default TLS version of SSLContextConfig to TLS 1.2+

Summary: Change the default minimum TLS version in wangle::SSLContextConfig to TLS 1.2.

Reviewed By: mingtaoy

Differential Revision: D26178470

fbshipit-source-id: 5dc46956a0ac443b31577b8d803e4bd1660f99fd

facebook/wangle,facebook/wangle,facebook/wangle