contracts: Replace cargo feature unstable-interface with config (#12787)

* Replace cargo feature with config

* Update frame/contracts/proc-macro/src/lib.rs

Co-authored-by: Sasha Gryaznov <hi@agryaznov.com>

Co-authored-by: Sasha Gryaznov <hi@agryaznov.com>

substrate/frame/contracts/src/benchmarking/sandbox.rs

@@ -64,7 +64,7 @@ where
 struct EmptyEnv;
 
 impl Environment<()> for EmptyEnv {
-	fn define(_store: &mut Store<()>, _linker: &mut Linker<()>) -> Result<(), LinkerError> {
+	fn define(_: &mut Store<()>, _: &mut Linker<()>, _: bool) -> Result<(), LinkerError> {
 		Ok(())
 	}
 }