Fix delivery of service desk thank you email with SMTP pool

What does this MR do?

After enabling SMTP pooling in production, we saw errors like: undefined method 'settings' for #<Mail::SMTPPool:0x00007f2e7bd09228>

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2603245

The service desk thank you email uses #deliver_later! which behaves differently to the regular #deliver_later.

This MR fixes the SMTP pool gem so that it handles #deliver_later! / #deliver!. This also changes the service desk mailer to use #deliver_later instead. Using the bang version of the method ignores the perform_deliveries setting and that is not intended.

Screenshots (strongly suggested)

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