Fix DMARC policy for list.pub.solar

Our _dmarc TXT record by default protects all subdomains, so an
adversary can't send mails on behalf of any subdomains (p=reject, p
stands for policy). We add a _dmarc.list TXT record with the same policy
to allow sending mails via list.pub.solar's postfix.
See https://dmarcly.com/blog/how-dmarc-works-with-subdomains-dmarc-sp-tag
for more context

terraform/lev-1/dns.tf

@@ -110,6 +110,11 @@ resource "namecheap_domain_records" "pub-solar" {
     type = "TXT"
     address = "v=DMARC1; p=reject;"
   }
+  record {
+    hostname = "_dmarc.list"
+    type = "TXT"
+    address = "v=DMARC1; p=reject;"
+  }
   record {
     hostname = "@"
     type = "MX"