Log user deactivations (#18541)

One liner to give us more clarity when auditing deactivations of user
accounts.

### Pull Request Checklist

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* [ ] Pull request is based on the develop branch
* [ ] 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`.".
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* [ ] [Code
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correct (run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

changelog.d/18541.misc

@@ -0,0 +1 @@
+Log user deactivations.

synapse/handlers/deactivate_account.py

@@ -96,6 +96,14 @@ class DeactivateAccountHandler:
                 403, "Deactivation of this user is forbidden", Codes.FORBIDDEN
             )
 
+        logger.info(
+            "%s requested deactivation of %s erase_data=%s id_server=%s",
+            requester.user,
+            user_id,
+            erase_data,
+            id_server,
+        )
+
         # FIXME: Theoretically there is a race here wherein user resets
         # password using threepid.