Spurious room upgrade warnings

I reloaded my dev element after the break and it's now screaming at me that my rooms are vulnerable and need upgrading which is because the js-sdk doesn't know about room version 6 (I hadn't started my local HS yet).

The short term fix is obviously teach the js-sdk about version 6 (https://github.com/matrix-org/matrix-js-sdk/pull/1572), but isn't the point of the string room versions is that the ones that are numeric are comparable to each other so a client that thinks version "5" is fine deduces that version "6" is also fine? Otherwise this is going to happen every time there's a new room version and we forget to tell the js-sdk about it.