* Remove dependency on `org.matrix.e2e_cross_signing` unstable feature
Currently, we have some code that relies on the server declaring support for an
`unstable_feature` called `org.matrix.e2e_cross_signing`. There is nothing in
the spec that requires this, so this would make us incompatible with some
server implementations.
The features in question were added in spec v1.1, so we can test for that
instead.
* fix unit test
6b451afc50