Update the default pre-join state to include the room topic, per MSC3173

This issue has been migrated from [#11186](https://github.com/matrix-org/synapse/issues/11186).

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[MSC3173](https://github.com/matrix-org/matrix-doc/pull/3173) updated the recommendation for which events to include in the stripped-state of a room to:

* Create event (`m.room.create`)
* Join rules (`m.room.join_rules`)
* Canonical alias (`m.room.canonical_alias`)
* Room avatar (`m.room.avatar`)
* Room name (`m.room.name`)
* Encryption information (`m.room.encryption`)
* Room topic (`m.room.topic`)

Synapse [currently sends](https://github.com/matrix-org/synapse/blob/28c68411028e15858819f2a5896313ce0e71c25b/synapse/config/api.py#L102-L110) all of these except `m.room.topic`.