- Update to 1.7.1.

This update fixes several majar security issues. Users are very strongly
recommended to update as soon as possible.

* Fix a bug which could cause room events to be incorrectly authorized using
events from a different room.
* Fix a bug causing responses to the /context client endpoint to not use the
pruned version of the event.
* Fix a cause of state resets in room versions 2 onwards.
* Fix a bug which could cause the federation server to incorrectly return
errors when handling certain obscure event graphs.

The full changelog is included in
/usr/share/doc/packages/matrix-synapse/CHANGES.md.