build(deps): update secp256k1 requirement from 0.15.0 to 0.16.0

Updates the requirements on [secp256k1](https://github.com/rust-bitcoin/rust-secp256k1) to permit the latest version.
- [Release notes](https://github.com/rust-bitcoin/rust-secp256k1/releases)
- [Changelog](https://github.com/rust-bitcoin/rust-secp256k1/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rust-bitcoin/rust-secp256k1/compare/0.15.0...0.16.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Cargo.toml

@@ -20,7 +20,7 @@ serde_json = "1.0"
 serde_with = "1.0"
 serde_bytes = "0.11.2"
 time = "*"
-secp256k1 = { version = "0.15.0", features = ["rand", "serde"] }
+secp256k1 = { version = "0.16.0", features = ["rand", "serde"] }
 sha2 = { version = "0.8", default-features = false }
 
 [lib]