K2: postpone "Enable DFA warnings" to version 1.9

#KT-50965 Fixed

compiler/testData/cli/jvm/firDfa.out

@@ -1,7 +1,4 @@
 warning: ATTENTION!
  This build uses experimental K2 compiler:
   -Xuse-k2
-compiler/testData/cli/jvm/firDfa.kt:3:10: warning: unnecessary non-null assertion (!!) on a non-null receiver of type kotlin/String
-        x!!.length
-         ^
 OK

compiler/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -257,7 +257,6 @@ enum class LanguageFeature(
     IgnoreNullabilityForErasedValueParameters(KOTLIN_1_8, kind = BUG_FIX),
     ForbidUsingExtensionPropertyTypeParameterInDelegate(KOTLIN_1_8, kind = BUG_FIX),
     SynchronizedSuspendError(KOTLIN_1_8),
-    EnableDfaWarningsInK2(KOTLIN_1_8, kind = OTHER), // KT-50965
     ReportNonVarargSpreadOnGenericCalls(KOTLIN_1_8, kind = BUG_FIX), // KT-48162
     RefineTypeCheckingOnAssignmentsToJavaFields(KOTLIN_1_8, kind = BUG_FIX),
     RangeUntilOperator(KOTLIN_1_8), // KT-15613
@@ -284,6 +283,7 @@ enum class LanguageFeature(
     ModifierNonBuiltinSuspendFunError(KOTLIN_1_9),
     BreakContinueInInlineLambdas(KOTLIN_1_9, defaultState = State.ENABLED), // KT-1436
     EnumEntries(KOTLIN_1_9, sinceApiVersion = ApiVersion.KOTLIN_1_8, defaultState = State.DISABLED), // KT-48872
+    EnableDfaWarningsInK2(KOTLIN_1_9, kind = OTHER), // KT-50965
 
     // Disabled for indefinite time. See KT-48535 and related discussion
     ApproximateIntegerLiteralTypesInReceiverPosition(sinceVersion = null),