[15.10] Add SMTP timeout configuration options

What does this MR do?

This backports !6874 (merged) to 15-10-stable.

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.

Relates to gitlab#410336 (closed)

Related issues

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