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/analysis-tests/testData/resolveWithStdlib/javaLangComparator.fir.txt

@@ -1,6 +1,6 @@
 FILE: javaLangComparator.kt
     public final fun test_2(list: R|kotlin/collections/List<kotlin/Int>|): R|kotlin/Unit| {
-        lval comp: R|java/util/Comparator<kotlin/Int>| = Q|java/util|.R|java/util/Comparator|<R|kotlin/Int|>(<L> = Comparator@fun <anonymous>(x: R|kotlin/Int!|, y: R|kotlin/Int!|): R|kotlin/Int| <inline=NoInline>  {
+        lval comp: R|java/util/Comparator<kotlin/Int!>| = Q|java/util|.R|java/util/Comparator|<R|kotlin/Int!|>(<L> = Comparator@fun <anonymous>(x: R|kotlin/Int!|, y: R|kotlin/Int!|): R|kotlin/Int| <inline=NoInline>  {
             ^ Int(1)
         }
         )