Use v3 API for crypto endpoints

[Velin92]

Currently we are using /_matrix/client/r0/keys/signatures/upload instead of the newer /_matrix/client/v3/keys/signatures/upload route

Original issue:
element-hq/element-ios#7557

[nisbet-hubbard]

@Velin92 I’ve created a diff for migrating to the v3 endpoint according to Matrix Spec v1.7. (I can’t do a pull request myself due to the requirement of contributing under legal name.)

Hope this is helpful.