Switch latest stable version to 1.6

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

@@ -357,18 +357,18 @@ enum class LanguageVersion(val major: Int, val minor: Int) : DescriptionAware {
             str.split(".", "-").let { if (it.size >= 2) fromVersionString("${it[0]}.${it[1]}") else null }
 
         @JvmField
-        val OLDEST_DEPRECATED = KOTLIN_1_3
+        val OLDEST_DEPRECATED = KOTLIN_1_4
 
         @JvmField
-        val FIRST_SUPPORTED = KOTLIN_1_4
+        val FIRST_SUPPORTED = KOTLIN_1_5
 
         @JvmField
-        val LATEST_STABLE = KOTLIN_1_5
+        val LATEST_STABLE = KOTLIN_1_6
     }
 }
 
 fun LanguageVersion.isStableOrReadyForPreview(): Boolean =
-    isStable || this == KOTLIN_1_6
+    isStable
 
 fun LanguageVersion.toKotlinVersion() = KotlinVersion(major, minor)