[K2/N] Fix property accessors modality in K2 metadata

^KT-56603

compiler/fir/fir-serialization/src/org/jetbrains/kotlin/fir/serialization/FirElementSerializer.kt

@@ -929,7 +929,8 @@ class FirElementSerializer private constructor(
         return Flags.getAccessorFlags(
             nonSourceAnnotations.isNotEmpty(),
             ProtoEnumFlags.visibility(normalizeVisibility(accessor)),
-            ProtoEnumFlags.modality(accessor.modality!!),
+            // non-default accessor modality is always final, so we check property.modality instead
+            ProtoEnumFlags.modality(property.modality!!),
             !isDefault,
             accessor.isExternal,
             accessor.isInline