chore: update const-hex@1.18

Author: cxw620

Cargo.lock

@@ -14,7 +14,7 @@ readme = "README.md"
 repository.workspace = true
 
 [dependencies]
-const-hex = { version = "1.16", default-features = false, features = ["alloc"], optional = true }
+const-hex = { version = "1.18", default-features = false, features = ["alloc"], optional = true }
 hpke-crypto = { version = "=0.0.1", path = "../hpke-crypto", default-features = false }
 serde = { version = "1.0", features = ["derive"], optional = true }
 serde_json = { version = "1.0", features = ["preserve_order"], optional = true }