MSC2334 - Change default room version to v5

Signed-off-by: Aaron Raimist <aaron@raim.ist>

proposals/2334-default-room-version-v5.md

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+# 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](https://github.com/matrix-org/matrix-doc/issues/2002).
+
+Room version 5 enforces the `valid_until_ts` timestamp on signing keys as proposed in [MSC2076](https://github.com/matrix-org/matrix-doc/issues/2076).
+
+## Proposal
+
+When [MSC2077](https://github.com/matrix-org/matrix-doc/issues/2077) 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.
+
+## Potential issues
+
+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.
+
+## Security considerations
+
+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.