Run background updates on all databases (#19181)

Fixes #18322 

This PR changes synapse startup to run background updates against all
databases instead of just the "main" database.
This follows [what the admin api
does](https://github.com/element-hq/synapse/blob/develop/synapse/rest/admin/background_updates.py#L71-L77).

See the above linked issue for further details of why this is
beneficial.

### Pull Request Checklist

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* [X] Pull request is based on the develop branch
* [X] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
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- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [X] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/19181.bugfix

@@ -0,0 +1 @@
+Run background updates on all databases.

synapse/app/homeserver.py

@@ -450,7 +450,8 @@ async def start(
     await _base.start(hs, freeze=freeze)
 
     # TODO: Feels like this should be moved somewhere else.
-    hs.get_datastores().main.db_pool.updates.start_doing_background_updates()
+    for db in hs.get_datastores().databases:
+        db.updates.start_doing_background_updates()
 
 
 def start_reactor(