fix(kubernetes): return kubernetes server group managers (#4583) (#4584)

(cherry picked from commit ccb864e)

Co-authored-by: Maggie Neterval <mneterval@google.com>
Co-authored-by: Ethan Rogers <ethanfrogers@users.noreply.github.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

clouddriver-web/src/main/groovy/com/netflix/spinnaker/clouddriver/controllers/ServerGroupManagerController.java

@@ -38,13 +38,13 @@
 @RestController
 @RequestMapping("/applications/{application}/serverGroupManagers")
 public class ServerGroupManagerController {
-  final List<ServerGroupManagerProvider<ServerGroupManager>> serverGroupManagerProviders;
+  final List<ServerGroupManagerProvider<? extends ServerGroupManager>> serverGroupManagerProviders;
 
   final RequestQueue requestQueue;
 
   @Autowired
   public ServerGroupManagerController(
-      List<ServerGroupManagerProvider<ServerGroupManager>> serverGroupManagerProviders,
+      List<ServerGroupManagerProvider<? extends ServerGroupManager>> serverGroupManagerProviders,
       RequestQueue requestQueue) {
     this.serverGroupManagerProviders = serverGroupManagerProviders;
     this.requestQueue = requestQueue;