synapse guest module: change license

Set license to multi-license of AGPL + Element Commercial, and add
associated license files to the repository root.

This does not add a notice of "This file includes modifications" to any
files because they were only moved and relicensed, but otherwise
unmodified.

The exception is run_in_venv.sh, which is a rewrite of upstream's
run_in_venv.js and thus lacks the original copyright entirely.

modules/restricted-guests/synapse/scripts/run_in_venv.sh

@@ -1,19 +1,8 @@
 #!/usr/bin/env sh
-
-# Copyright 2023 Nordeck IT + Consulting GmbH
-# Copyright 2025 New Vector Ltd.
+# Copyright (C) 2025 New Vector Ltd.
 #
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#     http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
+# SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+# Please see LICENSE files in the repository root for full details.
 
 # A script to run a command in a python virtual environment that is stored in
 # `<repo-root>/.venv`. If the virtual environment doesn't exist yet, it will