migrations: Match dependencies from 8.x branch.

The `zerver/0501_delete_dangling_usermessages` was backported to the
`8.x` branch (and the 8.3 release) in 3db1733310.  However, because
`main` contained migrations which `8.x` did not, it was backported
with a different `dependencies`:

```
    dependencies = [
        ("zerver", "0496_alter_scheduledmessage_read_by_sender"),
    ]
```

...as opposed to in `main`:

```
    dependencies = [
        ("zerver", "0500_realm_zulip_update_announcements_stream"),
    ]
```

This causes upgrades from 8.3 to `main` to fail:
```
django.db.migrations.exceptions.InconsistentMigrationHistory:
Migration zerver.0501_delete_dangling_usermessages is applied before
its dependency zerver.0500_realm_zulip_update_announcements_stream on
database 'default'.
```

Adjust the dependencies in `main` to match those in `8.x` where many
deploys will first have encountered the migration.

zerver/migrations/0501_delete_dangling_usermessages.py

@@ -196,7 +196,7 @@ class Migration(migrations.Migration):
     atomic = False
 
     dependencies = [
-        ("zerver", "0500_realm_zulip_update_announcements_stream"),
+        ("zerver", "0496_alter_scheduledmessage_read_by_sender"),
     ]
 
     operations = [