[15.10] Validate that SMTP settings do not enable both TLS and STARTTLS

What does this MR do?

This backports !6863 (merged) to 15-10-stable.

GitLab versions 15.10.4 and up shipped with gitlab!116925 (merged) in order to fix a Ruby 3 upgrade issue that made it impossible to disable STARTTLS if the SMTP server advertised support for it.

However, this change enforced the constraint that both SMTP TLS and STARTTLS cannot be enabled simultaneously. This follows the behavior of the net-smtp gem, which raises an exception if a user attempts to enable both.

This constraint was added because as described in https://github.com/mikel/mail/pull/1536, the logic for enabling/disabling TLS and/or STARTTLS is a bit tricky to get right and missed some important edge cases.

This commit adds a validation step that will throw an error if both settings appear:

gitlab_rails['smtp_tls'] = true
gitlab_rails['smtp_enable_starttls_auto'] = true

Previously if SMTP TLS were enabled, STARTTLS was disabled outright.

If gitlab_rails['smtp_tls'] is enabled, generally the easiest way to get things working is to set gitlab_rails['smtp_enable_starttls_auto'] to false.

Relates to gitlab#409835 (closed)

Also see !6858 (merged)

Related issues

Checklist

See Definition of done.

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Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
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• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

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 the GitLab Chart 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.