Update secp256k1 requirement from 0.27 to 0.28 (#109)

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
...

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

Cargo.toml

@@ -13,7 +13,7 @@ version = "0.10.0"
 zeroize = "1.0.0"
 rand = "0.8.0"
 rustc-hex = "2.0.1"
-secp256k1 = { version = "0.27", optional = true, features = ["recovery"] }
+secp256k1 = { version = "0.28", optional = true, features = ["recovery"] }
 serde = { version = "1.0", features = ["derive"]}
 
 # Libraries for for pure-rust crypto