Fix welcome campaign's list name

Pointed at a "Newsletter" list that doesn't exist in listmonk; the
real list name for this site is set by reground-infrastructure's
listmonk_client_lists (see listmonk-tools/sites.conf).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

campaigns/welcome/campaign.md

@@ -1,6 +1,6 @@
 ---
 subject: "Welcome to Thistle & Moth"
-lists: ["Newsletter"]
+lists: ["thistleandmoth-list"]
 from_email: "Thistle & Moth <hello@thistleandmoth.org>"
 tags: ["welcome"]
 ---