[FIR] Put new contract syntax under feauture flag

^KT-55171 Fixed

compiler/fir/analysis-tests/testData/resolveWithStdlib/contracts/fromSource/good/variousContracts/newSyntax/functionsWithContract.kt

@@ -1,3 +1,4 @@
+// LANGUAGE: +ContractSyntaxV2
 import kotlin.contracts.*
 
 fun test1(arg: Any?) contract [
@@ -19,4 +20,4 @@ fun test3(arg: Any?): Boolean contract [
 ] {
     require(arg != null)
     return true
-}
+}