Add SMTP read and write timeout values

What does this MR do?

In https://github.com/mikel/mail/pull/1427/files, mail v2.8.1 gem modified the default open and read timeout values to 5 seconds instead of the default Net::SMTP 30 and 60 seconds, respectively.

Since upgrading to GitLab 15.10.3, some users have observed receiving Net::ReadTimeout errors. This commit restores the original defaults and allows users to configure the timeouts.

Discourse came across a similar problem with the same solution. Some SMTP servers are slow to respond: https://meta.discourse.org/t/smtp-net-readtimeout-without-relation-to-net-or-login-problems-smtp-host-is-just-slow/235727

Related issues

Relates to gitlab-org/gitlab#410336 (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: gitlab-org/omnibus-gitlab!6874 (merged)
• 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.