Add 'private' to Cache-Control, match Rails expectations (#20608)

Several controlers set quite intricate Cache-Control headers in order to
hopefully not be cached by any intermediate proxies or local caches. Unfortunately,
these headers are processed by ActionDispatch::HTTP::Cache in a way that squashes
and discards any values set alongside no-store other than private:
8015c2c2cf/actionpack/lib/action_dispatch/http/cache.rb (L207-L209)

We want to preserve no-store on these responses, but we might as well remove
parts that are going to be dropped anyway. As many of the endpoints in these
controllers are private to a particular user, we should also add "private",
which will be preserved alongside no-store.

app/controllers/api/base_controller.rb

@@ -129,7 +129,7 @@ class Api::BaseController < ApplicationController
   end
 
   def set_cache_headers
-    response.headers['Cache-Control'] = 'no-cache, no-store, max-age=0, must-revalidate'
+    response.headers['Cache-Control'] = 'private, no-store'
   end
 
   def disallow_unauthenticated_api_access?