Update x25519-dalek requirement from 1.1 to 2.0

Updates the requirements on [x25519-dalek](https://github.com/dalek-cryptography/x25519-dalek) to permit the latest version.
- [Changelog](https://github.com/dalek-cryptography/x25519-dalek/blob/main/CHANGELOG.md)
- [Commits](https://github.com/dalek-cryptography/x25519-dalek/commits)

---
updated-dependencies:
- dependency-name: x25519-dalek
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -20,7 +20,7 @@ rand = "0.7"
 ring = { git = "https://github.com/rozbb/ring.git", branch = "master" }
 serde = { version = "1.0", features = ["derive"] }
 subtle = "2.1"
-x25519-dalek = "1.1"
+x25519-dalek = "2.0"
 
 [dev-dependencies]
 crossbeam = "0.8"