protocol: Reject X3DH PreKey messages

- Drop X3DH tests

- Drop cross-version tests with libsignal v0.12 and v0.21

- Change session benchmarks to use PQXDH, which is relevant if doing
  comparisons before/after this commit.

rust/protocol/cross-version-testing/tests/session.rs

@@ -56,8 +56,6 @@ fn test_basic_prekey() {
         &[
             || Box::new(LibSignalProtocolCurrent::new()),
             || Box::new(LibSignalProtocolV70::new()),
-            || Box::new(LibSignalProtocolV21::new()),
-            || Box::new(LibSignalProtocolV12::new()),
         ],
     );