DEV: Deprecate /posts/:id/reply-ids/all (#16136)

* DEV: Deprecate /posts/:id/reply-ids/all

It was added in ed4c0c4a63 and its only use was removed in b58867b6e9

Nothing in all-the* seems to be using this endpoint.

* Update app/controllers/posts_controller.rb

Co-authored-by: Alan Guo Xiang Tan <gxtan1990@gmail.com>

app/controllers/posts_controller.rb

@@ -309,6 +309,8 @@ class PostsController < ApplicationController
   end
 
   def all_reply_ids
+    Discourse.deprecate("/posts/:id/reply-ids/all is deprecated.", drop_from: "3.0")
+
     post = find_post_from_params
     render json: post.reply_ids(guardian, only_replies_to_single_post: false).to_json
   end