MSC2334 - Change default room version to v5
This MSC proposes changing the recommended default room version from v4 to v5. This implements steps 5 and 6 of the plan from MSC2002.
Room version 5 enforces the valid_until_ts timestamp on signing keys as
proposed in MSC2076.
When MSC2077 proposed room version 5, the default version continued to be v4 to allow old servers to continue to chat in newly created rooms. Server-Server API r0.1.2 (which contains the details on room version 5) and Synapse 1.0.0 (with support for room version 5) were both released more than 4 months ago which has given people plenty of time to update. Room version 5 should be the default room version so that newly created rooms enforce key-validity periods.
Servers which do not support room version 5 will not be able to participate in newly created rooms on other servers. Hopefully this will encourage them to update their server.
Room version 5 fixes known security vulnerabilities but that doesn't do much good if newly created rooms aren't using room version 5 by default.