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/testData/diagnostics/testsWithStdLib/resolve/ambiguousTypeReferences/qualifierForStaticCase.fir.txt

@@ -37,7 +37,7 @@ FILE: test2.kt
     }
 FILE: test3.kt
     public final fun test3(): R|ft<kotlin/collections/MutableMap<kotlin/String!, kotlin/String!>, kotlin/collections/Map<kotlin/String!, kotlin/String!>?>| {
-        ^test3 Q|java/util/Map|.R|java/util/Map.of*s|<R|kotlin/String|, R|kotlin/String|>()
+        ^test3 Q|java/util/Map|.R|java/util/Map.of*s|<R|kotlin/String!|, R|kotlin/String!|>()
     }
 FILE: test4.kt
     public final fun test4(): R|kotlin/Int| {