1. go-smtp is replaced by a fork that reverts StartTLS removal.
2. SASL LOGIN is no longer supported by upstream go-sasl, readded disabled by default.
3. Updated endpoint code to match new go-smtp authentication interfaces.
4. certmagic repo had some renames
5. Minimum Go version increased to 1.23 to match dependencies.
* use <h3> and <hr> elements to separate configuration directives
* use <code> elements to mark constants, fs paths,
module names etc.
* fix unneeded "\_" escaping
* fix list formatting
