Fix device list update stream ids going backward (#7158)

Occasionally we could get a federation device list update transaction which
looked like:

```
[
    {'edu_type': 'm.device_list_update', 'content': {'user_id': '@user:test', 'device_id': 'D2', 'prev_id': [], 'stream_id': 12, 'deleted': True}},
    {'edu_type': 'm.device_list_update', 'content': {'user_id': '@user:test', 'device_id': 'D1', 'prev_id': [12], 'stream_id': 11, 'deleted': True}},
    {'edu_type': 'm.device_list_update', 'content': {'user_id': '@user:test', 'device_id': 'D3', 'prev_id': [11], 'stream_id': 13, 'deleted': True}}
]
```

Having `stream_ids` which are lower than `prev_ids` looks odd. It might work
(I'm not actually sure), but in any case it doesn't seem like a reasonable
thing to expect other implementations to support.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/7158.misc b/changelog.d/7158.misc
new file mode 100644
index 0000000000..269b8daeb0
--- /dev/null
+++ b/changelog.d/7158.misc
@@ -0,0 +1 @@
+Fix device list update stream ids going backward.