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/resolve/extendedCheckers/canBeReplacedWithOperatorAssignment/flexibleTypeBug.fir.txt

@@ -1,6 +1,6 @@
 FILE: flexibleTypeBug.kt
     public final fun foo(): R|kotlin/Unit| {
-        lvar list1: R|ft<kotlin/collections/MutableList<kotlin/String!>, kotlin/collections/List<kotlin/String!>?>| = Q|java/util/Collections|.R|java/util/Collections.emptyList*s|<R|kotlin/String|>()
+        lvar list1: R|ft<kotlin/collections/MutableList<kotlin/String!>, kotlin/collections/List<kotlin/String!>?>| = Q|java/util/Collections|.R|java/util/Collections.emptyList*s|<R|kotlin/String!|>()
         lval list2: R|kotlin/collections/List<kotlin/String>| = R|kotlin/collections/listOf|<R|kotlin/String|>(String(b))
         R|<local>/list1| = R|<local>/list1|.R|kotlin/collections/plus|<R|kotlin/String!|>(R|<local>/list2|)
     }