move search reindex to schema 47

We're up to schema v47 on develop now, so this will have to go in there to have
an effect.

This might cause an error if somebody has already run it in the v46 guise, and
runs it again in the v47 guise, because it will cause a duplicate entry in the
bbackground_updates table. On the other hand, the entry is removed once it is
complete, and it is unlikely that anyone other than matrix.org has run it on
v46. The update itself is harmless to re-run because it deliberately copes with
the index already existing.

synapse/storage/schema/delta/38/postgres_fts_gist.sql

@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 
--- We no longer do this given we back it out again in schema 46
+-- We no longer do this given we back it out again in schema 47
 
 -- INSERT into background_updates (update_name, progress_json)
 --     VALUES ('event_search_postgres_gist', '{}');