Update secp256k1 requirement from 0.22 to 0.26 in /lightning-invoice

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] <support@github.com>

Author: dependabot[bot]

lightning-invoice/Cargo.toml

@@ -22,7 +22,7 @@ std = ["bitcoin_hashes/std", "num-traits/std", "lightning/std", "bech32/std"]
 [dependencies]
 bech32 = { version = "0.9.0", default-features = false }
 lightning = { version = "0.0.113", path = "../lightning", default-features = false }
-secp256k1 = { version = "0.24.0", default-features = false, features = ["recovery", "alloc"] }
+secp256k1 = { version = "0.26.0", default-features = false, features = ["recovery", "alloc"] }
 num-traits = { version = "0.2.8", default-features = false }
 bitcoin_hashes = { version = "0.11", default-features = false }
 hashbrown = { version = "0.8", optional = true }