Move to new mail relay server

Review https://github.com/pulibrary/pul_library_drupal/pull/2046. Confirm that the following is true after deploying to prod:

I can still send and receive emails
/var/log/mail on lib-ponyexpr-prod shows that the mails are sent successfully
For a rails app, make sure that enable_starttls is false. See https://github.com/pulibrary/orangelight/pull/4026 and https://github.com/pulibrary/pdc_describe/pull/1843 for examples.
For drupal only: The SMTP server needs to be updated manually here: https://library.princeton.edu/admin/config/system/smtp and the equivalent staging URL.

Please put your name next to each checkbox you decide to work on.

pulibrary / pul_library_drupal