Stabilize unsigned types KT-45653

Deprecate specialized unsigned iterators for removal.

Fix compiler tests:
- drop unsignedLiteralsOn1_2 because apiVersion 1.2 is no longer supported
- drop experimental unsigned literals diagnostic test

compiler/testData/diagnostics/tests/unsignedTypes/unsignedLiteralsOn1_2.fir.kt

@@ -1,6 +0,0 @@
-// !API_VERSION: 1.2
-
-val a = 0u
-val b = 1uL
-
-fun foo(u: UInt) {}

compiler/testData/diagnostics/tests/unsignedTypes/unsignedLiteralsOn1_2.kt

@@ -1,6 +0,0 @@
-// !API_VERSION: 1.2
-
-val a = <!UNSIGNED_LITERAL_WITHOUT_DECLARATIONS_ON_CLASSPATH!>0u<!>
-val b = <!UNSIGNED_LITERAL_WITHOUT_DECLARATIONS_ON_CLASSPATH!>1uL<!>
-
-fun foo(u: <!UNRESOLVED_REFERENCE!>UInt<!>) {}

compiler/testData/diagnostics/tests/unsignedTypes/unsignedLiteralsOn1_2.txt

@@ -1,5 +0,0 @@
-package
-
-public val a: [ERROR : Type cannot be resolved. Please make sure you have the required dependencies for unsigned types in the classpath]
-public val b: [ERROR : Type cannot be resolved. Please make sure you have the required dependencies for unsigned types in the classpath]
-public fun foo(/*0*/ u: [ERROR : UInt]): kotlin.Unit