Move EnableDfaWarningsInK2 into experimental section until K2/IDE EAP

This commit means that we will not see K2 DFA warnings until
we explicitly enable them (probably in 1.9.20 or 2.0).
We are going to enable DFA warnings at the moment when
K2/IDE is accessible for external users

compiler/testData/cli/jvm/firDfa.out

@@ -1,5 +1,2 @@
 warning: language version 2.0 is experimental, there are no backwards compatibility guarantees for new language and library features
-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

@@ -272,7 +272,6 @@ enum class LanguageFeature(
     SkipStandaloneScriptsInSourceRoots(KOTLIN_1_9, kind = OTHER), // KT-52525
     ModifierNonBuiltinSuspendFunError(KOTLIN_1_9, kind = BUG_FIX), // KT-49264
     EnumEntries(KOTLIN_1_9, sinceApiVersion = ApiVersion.KOTLIN_1_8, kind = UNSTABLE_FEATURE), // KT-48872
-    EnableDfaWarningsInK2(KOTLIN_1_9, kind = OTHER), // KT-50965
     ForbidSuperDelegationToAbstractFakeOverride(KOTLIN_1_9, kind = BUG_FIX), // KT-49017
     DataObjects(KOTLIN_1_9), // KT-4107
     ProhibitAccessToEnumCompanionMembersInEnumConstructorCall(KOTLIN_1_9, kind = BUG_FIX), // KT-49110
@@ -325,7 +324,7 @@ enum class LanguageFeature(
     JsAllowImplementingFunctionInterface(sinceVersion = null, kind = UNSTABLE_FEATURE),
     CustomEqualsInValueClasses(sinceVersion = null, kind = OTHER), // KT-24874
     InlineLateinit(sinceVersion = null, kind = OTHER), // KT-23814
-
+    EnableDfaWarningsInK2(sinceVersion = null, kind = OTHER), // KT-50965
     ;
 
     init {