fix(nlc): add deprecation warning

add deprecation warning to NLC constructor

fix #9626

src/IBM.Watson.NaturalLanguageClassifier.v1/NaturalLanguageClassifierService.cs

@@ -34,6 +34,11 @@
 
 namespace IBM.Watson.NaturalLanguageClassifier.v1
 {
+    [System.Obsolete("On 9 August 2021, IBM announced the deprecation of the Natural Language Classifier service." +
+        "The service will no longer be available from 8 August 2022. As of 9 September 2021, you will not be able to create new instances." +
+        "\nExisting instances will be supported until 8 August 2022. Any instance that still exists on that date will be deleted." +
+        "\nFor more information, see https://cloud.ibm.com/docs/natural-language-classifier?topic=natural-language-classifier-migrating")]
+
     public partial class NaturalLanguageClassifierService : IBMService, INaturalLanguageClassifierService
     {
         const string defaultServiceName = "natural_language_classifier";