Fix SMTP when TLS is disabled

What does this MR do?

This commit fixes mail sending when TLS is disabled. In Ruby 3.0.5, net-smtp v0.2.1 enabled TLS by default if the server advertises STARTTLS support. However, mail v2.7.1 didn't explicitly disable TLS (https://github.com/mikel/mail/issues/1434), so TLS may be used with Ruby 3 even if it is disabled. mail v2.8.1 has since fixed this issue via https://github.com/mikel/mail/pull/1480.

However, mail v2.8.1 has a bug in the logic for retrieving the settings (https://github.com/mikel/mail/blob/2.8.1/lib/mail/network/delivery_methods/smtp.rb#L114):

tls = settings[:tls] || settings[:ssl]

If settings[:tls] is false and settings[:ssl] is nil, then the result of false || nil is nil.

This means that TLS cannot be disabled if settings[:tls] is set to false.

To fix this, just add a redundant ssl config parameter.

Related issues

This came out of gitlab-org/gitlab#399241 (closed).

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.