Deprecate DiscoveryPlugin#getDiscoveryTypes (elastic#38410)

Marks `DiscoveryPlugin#getDiscoveryTypes` as deprecated since we intend to
remove the ability for plugins to provide their own discovery type in a future
version.

Relates elastic#38414

Author: DaveCTurner

server/src/main/java/org/elasticsearch/plugins/DiscoveryPlugin.java

@@ -66,7 +66,10 @@ public interface DiscoveryPlugin {
      * @param clusterApplier Use to locally apply cluster state updates
      * @param clusterSettings Use to get cluster settings
      * @param hostsProvider Use to find configured hosts which should be pinged for initial discovery
+     *
+     * @deprecated Future versions will not support pluggable discovery implementations.
      */
+    @Deprecated
     default Map<String, Supplier<Discovery>> getDiscoveryTypes(ThreadPool threadPool, TransportService transportService,
                                                                NamedWriteableRegistry namedWriteableRegistry,
                                                                MasterService masterService,