Change from nashmi/smtp to the more maintained ixdotai/smtp

[yanokwa]

This PR introduces the maintained, drop-in replacement for namshi/smtp. namshi/docker-smtp#81 (comment)

I tested this by starting with a v2023.1.0 install, changing to this container, rebuilding, and sending a password reset email through an AWS SMTP server. I've also tested sending a password reset email through the server itself.

The reason for ignoreTLS is that exim now generates a self-signed cert and advertises itself as a TLS-enabled server. nodemailer, the Central library that sends the emails, doesn't like self-signed certs, so that's why there is a few flag.

Alternatives I rejected:

• add MAIN_TLS_ENABLE = false to _docker_additional_macros fails because TLS support is compiled in. https://www.exim.org/exim-html-current/doc/html/spec_html/ch-main_configuration.html suggests there is no way to turn it off.
• use rejectUnauthorized in docker-compose. It's harder to add/remove and resolve conflicts. It feels clearer to ignore all of TLS and tell people to remove that line when using a custom mail client.
• build my own SMTP image. Seemed unwise to add to our maintenance burden.

I did not test a DKIM setup, but we were on exim 4.92 and we are now on exim 4.97. I don't expect any breaking changes and https://github.com/Exim/exim/blob/master/doc/doc-txt/ChangeLog doesn't suggest that there are. Also, I doubt a lot of people use the DKIM option.

[lognaturel]

Ignoring TLS seems reasonable to me given the options. I do think we want to be on a supported image.

[lognaturel]

Merging now so we can build on this.