chore(deps): bump crypto_kx to 0.2.1

[mrain]

Description

closes: #363

$ cargo update
$ cargo tree | grep curve25519                                                                                                                                                  [20:49:29]
│   │   ├── curve25519-dalek v4.0.0

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• Targeted PR against correct branch (main)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer