Update secp256k1 requirement from 0.21 to 0.26

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/secp256k1-0.26.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

ethcontract/Cargo.toml

@@ -40,7 +40,7 @@ hex = "0.4"
 jsonrpc-core = "18.0"
 lazy_static = "1.4"
 primitive-types = { version = "0.10", features = ["fp-conversion"] }
-secp256k1 = { version = "0.21", features = ["recovery"] }
+secp256k1 = { version = "0.26", features = ["recovery"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 thiserror = "1.0"