K2: repeat K1 representation for flexible type parameters

This commit changes the behavior of KT-59138 effectively declining it in 2.0.
However, we plan to implement KT-59138 behavior under a feature
flag in 2.0 (see KT-66447), and switch this feature on version 2.x.

Also, this commit implements the LC resolution about postponing
KT-57014 change. We don't have KT-57014 described behavior in 2.0 anymore.
However, we plan to implement a deprecation warning here, see KT-65578.

After this commit, 6 diagnostic tests become incorrectly broken:
- 5 tests from PurelyImplementedCollection group
- a test platformTypes/nullableTypeArgument.kt

This commit also breaks currently fixed-in-k2 KT-50134
(it is fixed again in the following commits),
as well as KT-58933 (it will remain not fixed till we enable KT-59138
behavior again).

#KT-65596 In Progress
#KT-57014 In Progress
#KT-58933 Submitted

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/JavaTypeConversion.kt

@@ -110,19 +110,7 @@ private fun JavaType?.toConeTypeProjection(
             }
             val upperBound = toConeKotlinTypeForFlexibleBound(session, javaTypeParameterStack, mode, attributes, lowerBound)
 
-            val finalLowerBound = when (lowerBound) {
-                is ConeTypeParameterType ->
-                    ConeDefinitelyNotNullType.create(
-                        lowerBound, session.typeContext,
-                        // Upper bounds might be not initialized properly yet, so we force creating DefinitelyNotNullType
-                        // It should not affect semantics, since it would be still a valid type anyway
-                        avoidComprehensiveCheck = true,
-                    ) ?: lowerBound
-
-                else -> lowerBound
-            }
-
-            if (isRaw) ConeRawType.create(finalLowerBound, upperBound) else ConeFlexibleType(finalLowerBound, upperBound)
+            if (isRaw) ConeRawType.create(lowerBound, upperBound) else ConeFlexibleType(lowerBound, upperBound)
         }
 
         is JavaArrayType -> {