Add a schema delta to drop unstable private read receipts. (#13692)

Otherwise they'll be leaked due to the filtering code only respecting
the stable identifiers for private read receipts.

changelog.d/13692.removal

+Remove support for unstable [private read receipts](https://github.com/matrix-org/matrix-spec-proposals/pull/2285).

synapse/storage/schema/main/delta/72/05remove_unstable_private_read_receipts.sql

+/* Copyright 2022 The Matrix.org Foundation C.I.C
+ *
+ * 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.
+ */
+
+-- Drop previously received private read receipts so they do not accidentally
+-- get leaked to other users.
+DELETE FROM receipts_linearized WHERE receipt_type = 'org.matrix.msc2285.read.private';
+DELETE FROM receipts_graph WHERE receipt_type = 'org.matrix.msc2285.read.private';