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/loadJava/compiledJava/MethodReferencesOuterClassTP.fir.txt

@@ -2,7 +2,7 @@ public final class MethodReferencesOuterClassTP<P : R|kotlin/Any!|> : R|kotlin/A
     public constructor<P : R|kotlin/Any!|>(): R|test/MethodReferencesOuterClassTP<P>|
 
     public final inner class Inner<Outer(P) : R|kotlin/Any!|> : R|kotlin/Any| {
-        public final fun <Q : R|ft<P & Any, P?>|> f(): R|kotlin/Unit|
+        public final fun <Q : R|P!|> f(): R|kotlin/Unit|
 
         public test/MethodReferencesOuterClassTP<P>.constructor(): R|test/MethodReferencesOuterClassTP.Inner<P>|