• update to 3.6-20200720

Incompatible change with snapshot 20200705

The minimum OpenSSL version is 1.1.1, which will reach the end
of life by 2023-09-11.

The default digest has changed from md5 to sha256 (Postfix 3.6 with
compatibility_level >= 3). With a lower compatibility_level setting,
Postfix defaults to using md5, and logs a warning when a Postfix
configuration specifies no explicit digest type.

Export-grade Diffie-Hellman key exchange is no longer supported,
and the tlsproxy_tls_dh512_param_file parameter is ignored,