build(deps): update secp256k1 requirement from 0.19.0 to 0.20.0 (#496)

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/commits)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Andronik Ordian <write@reusable.software>

parity-crypto/Cargo.toml

@@ -26,7 +26,7 @@ rand = "0.7.3"
 ripemd160 = "0.9.1"
 rustc-hex = { version = "2.1.0", default-features = false, optional = true }
 scrypt = { version = "0.5.0" }
-secp256k1 = { version = "0.19.0", optional = true, features = ["global-context", "recovery", "rand-std"] }
+secp256k1 = { version = "0.20.0", optional = true, features = ["global-context", "recovery", "rand-std"] }
 sha2 = "0.9.2"
 subtle = "2.4.0"
 tiny-keccak = { version = "2.0.2", features = ["keccak"] }