DEV: Fix SMTP bounce regexp (#16019)

Never trust me with regexp. Follow up to
01ef1d08fc,
which did not take into account codes in
the format X.X.XX (with the 2 digits on the end)

db/post_migrate/20220220234155_conform_bounce_error_code.rb

@@ -2,7 +2,7 @@
 #
 class ConformBounceErrorCode < ActiveRecord::Migration[6.1]
   def up
-    DB.exec(<<~SQL, regexp: '\d\.\d\.\d|\d\d\d')
+    DB.exec(<<~SQL, regexp: '\d\.\d\.\d+|\d{3}')
       UPDATE email_logs
       SET bounce_error_code = (
         SELECT array_to_string(