How do I convert a "Not Secure" website in Safari to secure?

When Safari 13.1 and later detects that a website is running deprecated Transport Layer Security (TLS) versions 1.0 or 1.1, it will flag the site as "not secure." There is nothing you can do as the remote site administrators need to get on the ball and update their TLS security.

From the Safari 13.1 release notes:

You use https rather than http to access a secure site. But the site must be secure first. Just using https does not make a site secure it serves a secure site securely.