FIX: Conform EmailLog#bounce_error_code to RFC (#16010)

This commit makes sure that the email log's bounce_error_code
conforms to the SMTP error code RFC on save, so that
it is always in the format X.X.X or XXX without any
additional string details. Also included is a migration
to fix this issue for past records.

db/post_migrate/20220220234155_conform_bounce_error_code.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+#
+class ConformBounceErrorCode < ActiveRecord::Migration[6.1]
+  def up
+    DB.exec(<<~SQL, regexp: '\d\.\d\.\d|\d\d\d')
+      UPDATE email_logs
+      SET bounce_error_code = (
+        SELECT array_to_string(
+          regexp_matches(bounce_error_code, :regexp),
+          ''
+        )
+      ) WHERE bounce_error_code IS NOT NULL;
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end