Contracts: Stabilize XCM host fns (#4213)

See 
https://github.com/paritytech/ink/pull/1912
https://github.com/paritytech/ink-docs/pull/338

substrate/frame/contracts/src/wasm/runtime.rs

@@ -2104,7 +2104,6 @@ pub mod env {
 
 	/// Execute an XCM program locally, using the contract's address as the origin.
 	/// See [`pallet_contracts_uapi::HostFn::execute_xcm`].
-	#[unstable]
 	fn xcm_execute(
 		ctx: _,
 		memory: _,
@@ -2143,7 +2142,6 @@ pub mod env {
 
 	/// Send an XCM program from the contract to the specified destination.
 	/// See [`pallet_contracts_uapi::HostFn::send_xcm`].
-	#[unstable]
 	fn xcm_send(
 		ctx: _,
 		memory: _,