[pubsub] Mark publish and subscribe features as recommended (not required)

As per recent standards@ discussion, these features are not required,
but recommended. See also

- xsf/xeps#1467
- https://logs.xmpp.org/xsf/2025-10-03#2025-10-03-f173fd843da7c3ee
- https://mail.jabber.org/hyperkitty/list/standards@xmpp.org/thread/R4K2OAPVBQ4IC757F2YBZDV4PPHFGRNS/

Author: Flowdalic

smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/PubSubFeature.java

@@ -1,6 +1,6 @@
 /*
  *
- * Copyright 2015 Florian Schmaus
+ * Copyright 2015-2025 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -56,7 +56,7 @@ public enum PubSubFeature implements CharSequence {
     persistent_items(Support.recommended),
     presence_notifications(Support.optional),
     presence_subscribe(Support.recommended),
-    publish(Support.required),
+    publish(Support.recommended),
     publish_options(Support.optional),
     publish_only_affiliation(Support.optional),
     publisher_affiliation(Support.recommended),
@@ -67,7 +67,7 @@ public enum PubSubFeature implements CharSequence {
     retrieve_default_sub(Support.optional),
     retrieve_items(Support.recommended),
     retrieve_subscriptions(Support.recommended),
-    subscribe(Support.required),
+    subscribe(Support.recommended),
     subscription_options(Support.optional),
     subscriptions_notifications(Support.optional),
     instant_nodes(Support.recommended),