change the envelope-from on subscription request confirmations in newsletter

in https://gitlab.torproject.org/tpo/tpa/team/-/issues/40286 (confidential issue), @smith reported receiving a significant number of bounces at info@tpo. after investigation, it turns out those are subscription attempts that bounce. here's a part of those messages:

Subject: Please confirm your subscription to The Tor Project newsletter.
From: The Tor Project <info@torproject.org>
Message-ID: 86cedbe76ed1b59e257095939503a38c@donate-api.torproject.org
X-Mailer: PHPMailer 5.2.27 (https://github.com/PHPMailer/PHPMailer)

the envelope-from (AKA the Return-Path header or MAIL FROM command in SMTP) of those messages is also set to info@torproject.

this should be something that is capable of handling those bounces, or devnull.

@eric, can you take this on?