Update Rust crate secp256k1 to 0.31

Author: renovate[bot]

Cargo.lock

@@ -49,7 +49,7 @@ bip32 = { version = "=0.6.0-pre.1", default-features = false, features = [
 block-buffer = { version = "=0.11.0-rc.3" }
 crypto-common = { version = "=0.2.0-rc.1" }
 ripemd = { version = "0.1", default-features = false }
-secp256k1 = { version = "0.29", default-features = false, features = ["alloc"] }
+secp256k1 = { version = "0.31", default-features = false, features = ["alloc"] }
 transparent = { package = "zcash_transparent", version = "0.6", path = "external/librustzcash/zcash_transparent", default-features = false }
 zcash_script = { version = "0.4.2", default-features = false }
 zeroize = { version = "1.7", default-features = false }