[FIR] Put new contract syntax under feauture flag

^KT-55171 Fixed

compiler/testData/diagnostics/testsWithStdLib/contracts/newSyntax/complexContractDescription.kt

@@ -1,3 +1,4 @@
+// LANGUAGE: +ContractSyntaxV2
 // !DIAGNOSTICS: -UNUSED_VARIABLE
 
 import kotlin.contracts.*
@@ -21,4 +22,4 @@ fun bar(arg: Any?, block: () -> Int): Boolean contract <!UNSUPPORTED!>[
         return true
     }
     return false
-}
+}