[KxSerialization] Fix SERIALIZER_NOT_FOUND diagnostic for enums

If enum class from dependencies is used, which was compiled by a new version of the plugin, which uses a factory and does not create a nested `$serializer` class, then an `SERIALIZER_NOT_FOUND` diagnostic error is thrown for such an enumeration.
This happens if the old serialization runtime is used for the current project - in this case, the serializer is taken from the nested class `$serializer`. Since it is missing, the diagnostics does not work correctly.
It is acceptable for enumerations to ignore this error, because we know that enumerations are always serializable.

Merge-request: KT-MR-8818
Merged-by: Sergey Shanshin <Sergey.Shanshin@jetbrains.com>

plugins/kotlinx-serialization/kotlinx-serialization.k2/src/org/jetbrains/kotlinx/serialization/compiler/fir/checkers/FirSerializationPluginClassChecker.kt

@@ -539,7 +539,10 @@ object FirSerializationPluginClassChecker : FirClassChecker() {
             }
             checkTypeArguments(type, source, reporter)
         } else {
-            reporter.reportOn(source, FirSerializationErrors.SERIALIZER_NOT_FOUND, type)
+            if (!type.isEnum) {
+                // enums are always serializable
+                reporter.reportOn(source, FirSerializationErrors.SERIALIZER_NOT_FOUND, type)
+            }
         }
     }