fix correct minimal versions for serde crates

Author: apoelstra

Cargo.toml

@@ -38,7 +38,7 @@ global-context-less-secure = ["global-context"]
 
 [dependencies]
 secp256k1-sys = { version = "0.8.1", default-features = false, path = "./secp256k1-sys" }
-serde = { version = "1.0", default-features = false, optional = true }
+serde = { version = "1.0.103", default-features = false, optional = true }
 
 # You likely only want to enable these if you explicitly do not want to use "std", otherwise enable
 # the respective -std feature e.g., bitcoin-hashes-std
@@ -48,7 +48,7 @@ rand = { version = "0.8", default-features = false, optional = true }
 [dev-dependencies]
 rand_core = "0.6"
 serde_cbor = "0.10.0"
-serde_test = "1.0"
+serde_test = "1.0.19"
 bincode = "1.3.3"
 
 [target.wasm32-unknown-unknown.dev-dependencies]