MEDIUM: ssl: use TLSv1.2 as the minimum default on bind lines

MEDIUM: ssl: use TLSv1.2 as the minimum default on bind lines

Since HAProxy 1.8, the TLS default minimum version was set to TLSv1.0 to
avoid using the deprecated SSLv3.0. Since then, the standard changed and
the recommended TLS version is now TLSv1.2.

This patch changes the minimum default version to TLSv1.2 on bind lines.
If you need to use prior TLS version, this is still possible by
using the ssl-min-ver keyword.