Merge tag 'v1.38.1' into release-v1.39

Synapse 1.38.1 (2021-07-22)
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Bugfixes
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- Always include `device_one_time_keys_count` key in `/sync` response to work around a bug in Element Android that broke encryption for new devices. ([\#10457](https://github.com/matrix-org/synapse/issues/10457))