Add license information to files in matrix_playbook_migration/

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

roles/custom/matrix_playbook_migration/tasks/migrate_matrix_mailer.yml

@@ -1,3 +1,7 @@
+# SPDX-FileCopyrightText: 2024 Slavi Pantaleev
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
 ---
 
 # This migrates the mailer from the old path (`/matrix/mailer`) to the new path (`/matrix/exim-relay`, controlled by `exim_relay_base_path`),