Update minimum SSL version

Update minimum SSL version

Change default of ssl_min_protocol_version to TLSv1.2 (from TLSv1,
which means 1.0).  Older versions are still supported, just not by
default.

TLS 1.0 is widely deprecated, and TLS 1.1 only slightly less so.  All
OpenSSL versions that support TLS 1.1 also support TLS 1.2, so there
would be very little reason to, say, set the default to TLS 1.1
instead on grounds of better compatibility.

The test suite overrides this new setting, so it can still run with
older OpenSSL versions.

Discussion: https://www.postgresql.org/message-id/flat/b327f8df-da98-054d-0cc5-b76a857cfed9%402ndquadrant.com