Bump secp256k1 from 0.20.3 to 0.21.2 (#3326)

Bumps [secp256k1](https://github.com/rust-bitcoin/rust-secp256k1) from 0.20.3 to 0.21.2.
- [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)

---
updated-dependencies:
- dependency-name: secp256k1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

zebra-chain/Cargo.toml

@@ -36,7 +36,7 @@ jubjub = "0.8.0"
 lazy_static = "1.4.0"
 rand_core = "0.6"
 ripemd160 = "0.9"
-secp256k1 = { version = "0.20.3", features = ["serde"] }
+secp256k1 = { version = "0.21.2", features = ["serde"] }
 serde = { version = "1", features = ["serde_derive", "rc"] }
 serde-big-array = "0.3.2"
 sha2 = { version = "0.9.8", features=["compress"] }