Fix non-strings in the event_search table (#12037)

Don't attempt to add non-string `value`s to `event_search` and add a
background update to clear out bad rows from `event_search` when
using sqlite.

Signed-off-by: Sean Quah <seanq@element.io>

synapse/storage/schema/main/delta/68/05_delete_non_strings_from_event_search.sql.sqlite

@@ -0,0 +1,22 @@
+/* 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.
+ */
+
+
+-- Delete rows with non-string `value`s from `event_search` if using sqlite.
+--
+-- Prior to Synapse 1.44.0, malformed events received over federation could
+-- cause integers to be inserted into the `event_search` table.
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+  (6805, 'event_search_sqlite_delete_non_strings', '{}');