[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/firProblems/SameJavaFieldReferences.fir.kt.txt

@@ -0,0 +1,4 @@
+fun foo() {
+  val ref1: KProperty0<@FlexibleNullability String?> = SomeJavaClass::someJavaField
+  val ref2: KProperty0<@FlexibleNullability String?> = SomeJavaClass::someJavaField
+}