mail/postfix-current: Update to 3.7-20210707

• Update to 3.7-20210707
• Check if OpenSSL at least 1.1.1a is being used
• LibreSSL is not supported as it dos not (yet) provide the TLSv1.3 API found in OpenSSL 1.1.1 API.

(cherry picked from commit 792ca3684227788b8249d54e8bdd0e27f5590008)