[FIR] Don't enhance Int and String Java final static fields

It's effectively a breaking change (^KT-62558)

K2 assigns flexible type to all static fields (from Java) and, for example,
`String? becomes String unlike K1. It affects IR signature generating.
That's why signature dump is disabled for some tests.

^KT-57811 Fixed
^KT-61786 Fixed

compiler/testData/ir/irText/declarations/kt29833.fir.kt.txt

@@ -7,11 +7,11 @@ object Definitions {
 
   }
 
-  const val KT_CONSTANT: String
+  const val KT_CONSTANT: @FlexibleNullability String?
     field = "constant"
     get
 
-  val ktValue: String
+  val ktValue: @FlexibleNullability String?
     field = #CONSTANT
     get