[FIR] Make redundant visibility modifier a warning

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrors.kt

@@ -46,7 +46,6 @@ object FirErrors {
     val SUPERCLASS_NOT_ACCESSIBLE_FROM_INTERFACE by error0<FirSourceElement, PsiElement>()
     val ABSTRACT_SUPER_CALL by error0<FirSourceElement, PsiElement>()
     val QUALIFIED_SUPERTYPE_EXTENDED_BY_OTHER_SUPERTYPE by error0<FirSourceElement, PsiElement>()
-    val REDUNDANT_VISIBILITY_MODIFIER by error0<FirSourceElement, PsiElement>()
 
     val INAPPLICABLE_INFIX_MODIFIER by existing<FirSourceElement, PsiElement, String>(Errors.INAPPLICABLE_INFIX_MODIFIER)
     val CONSTRUCTOR_IN_OBJECT by existing<FirSourceElement, KtDeclaration>(Errors.CONSTRUCTOR_IN_OBJECT)
@@ -82,6 +81,9 @@ object FirErrors {
 
     // Control flow diagnostics
     val UNINITIALIZED_VARIABLE by error1<FirSourceElement, PsiElement, FirPropertySymbol>()
+
+    // Extended checkers group
+    val REDUNDANT_VISIBILITY_MODIFIER by warning0<FirSourceElement, PsiElement>()
 }