Add DB delta to remove the old state group deletion job (#18284)

This background DB delta removes the old state group deletion background
update from the `background_updates` table if it exists.
The `delete_unreferenced_state_groups_bg_update` update should only
exist in that table if a homeserver ran v1.126.0rc1/v1.126.0rc2, and
rolled back or forward to any other version of Synapse before letting
the update finish.

### 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`.".
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* [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/18284.misc

@@ -0,0 +1 @@
+Add DB delta to remove the old state group deletion job.

synapse/storage/schema/state/delta/90/03_remove_old_deletion_bg_update.sql

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+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2025 New Vector, Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+-- Remove the old unreferenced state group deletion background update if it exists
+DELETE FROM background_updates WHERE update_name = 'delete_unreferenced_state_groups_bg_update';