Update secp256k1 requirement from 0.27 to 0.28 (#708)

Updates the requirements on [secp256k1](https://github.com/rust-bitcoin/rust-secp256k1) to permit the latest version.
- [Changelog](https://github.com/rust-bitcoin/rust-secp256k1/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rust-bitcoin/rust-secp256k1/commits/secp256k1-0.28.0)

---
updated-dependencies:
- dependency-name: secp256k1
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Cargo.toml

@@ -29,7 +29,7 @@ serde_json = "1.0.39"
 tiny-keccak = { version = "2.0.1", features = ["keccak"] }
 pin-project = "1.0"
 # Optional deps
-secp256k1 = { version = "0.27", features = ["recovery"], optional = true }
+secp256k1 = { version = "0.28", features = ["recovery"], optional = true }
 once_cell = { version = "1.8.0", optional = true }
 
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