This commit fixes mail sending when TLS is disabled. In Ruby 3.0.5,
net-smtp v0.2.1 enabled TLS by default. However, mail v2.7.1 didn't
explicitly disable TLS (https://github.com/mikel/mail/issues/1434), so
TLS is enabled by default with Ruby 3. mail v2.8.1 has since fixed
this issue via https://github.com/mikel/mail/pull/1536.

This commit also removes one monkey patch introduced in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24153/diffs
since https://github.com/mikel/mail/pull/1210 was merged for mail
v2.8.0.

Relates to:

1. https://gitlab.com/gitlab-org/gitlab/-/issues/399241
2. https://gitlab.com/gitlab-org/gitlab/-/issues/396252

Changelog: fixed