Use v3 API for crypto endpoints

Currently, [element uses](https://github.com/vector-im/element-ios/issues/3114) `/_matrix/client/r0/keys/signatures/upload` instead of the newer and correct `/_matrix/client/v3/keys/signatures/upload` route. As a result, attempts to set up key backup on the Conduit implementation of Matrix, for example, throw errors.

This has also been documented and fixed for FluffyChat: https://gitlab.com/famedly/fluffychat/-/issues/986.

Start by locating the Element iOS request that uses /_matrix/client/r0/keys/signatures/upload and compare it with the v3 route described in the issue. Verify the key-backup request against a Conduit Matrix server; done means the v3 endpoint is used and setup no longer fails. An open linked pull request already addresses this issue.

Written by the indexing model from the issue text.