fix minimal versions of serde

Author: apoelstra

Cargo.toml

@@ -32,12 +32,12 @@ bitcoin_hashes = "0.11.0"
 # Used for ContractHash::from_json_contract.
 serde_json = { version = "1.0", optional = true }
 
-actual-serde = { package="serde", version = "1.0", features=["derive"], optional = true }
+actual-serde = { package="serde", version = "1.0.103", features=["derive"], optional = true }
 
 [dev-dependencies]
 rand = "0.8"
 rand_chacha = "0.3"
-serde_test = "1.0"
+serde_test = "1.0.19"
 serde_json = "1.0"
 serde_cbor = "0.8"   # older than latest version to support 1.41.1
 bincode = "1.3"