Delete unreferenced state groups in background (#18254)

This PR fixes #18154 to avoid de-deltaing state groups which resulted in
DB size temporarily increasing until the DB was `VACUUM`'ed. As a
result, less state groups will get deleted now.
It also attempts to improve performance by not duplicating work when
processing state groups it has already processed in previous iterations.

### Pull Request Checklist

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---------

Co-authored-by: Erik Johnston <erikj@element.io>

1 files changed, 1 insertions, 1 deletions

diff --git a/docs/development/database_schema.md b/docs/development/database_schema.md

index 37a06acc12..620d1c16b0 100644
--- a/docs/development/database_schema.md
+++ b/docs/development/database_schema.md
@@ -162,7 +162,7 @@ by a unique name, the current status (stored in JSON), and some dependency infor
 * Whether the update requires a previous update to be complete.
 * A rough ordering for which to complete updates.
 
-A new background updates needs to be added to the `background_updates` table:
+A new background update needs to be added to the `background_updates` table:
 
 ```sql
 INSERT INTO background_updates (ordering, update_name, depends_on, progress_json) VALUES