[FE] Always infer upper types to intersection types in K2...

...and always use old algorithm for K1

^KT-51221

compiler/fir/providers/src/org/jetbrains/kotlin/fir/types/ConeInferenceContext.kt

@@ -577,4 +577,7 @@ interface ConeInferenceContext : TypeSystemInferenceExtensionContext, ConeTypeCo
 
     override fun createSubstitutorForSuperTypes(baseType: KotlinTypeMarker): TypeSubstitutorMarker? =
         if (baseType is ConeLookupTagBasedType) createSubstitutionForSupertype(baseType, session) else null
+
+    override val isK2: Boolean
+        get() = true
 }