FEATURE: Use new hashtag autocomplete system on all sites (#21788)

Followup to:

* 7c975481597c46ba1064b37f8f4627da82c5b689
* 0b3cf83e3c7d03c2dcd54f86de60efe32f385a44
* eae47d82e22189646ce1b29ea2f3b5b2f8b64312

This commit enables the no-longer-experimental hashtag autocomplete changes
for all sites. It is now at a point where it is ready to be promoted
to the only way, and following this commit all old hashtag code
will be deleted

c.f. https://meta.discourse.org/t/hashtags-are-getting-a-makeover/248866

config/site_settings.yml

@@ -2090,6 +2090,7 @@ developer:
   enable_experimental_hashtag_autocomplete:
     default: true
     client: true
+    hidden: true
   experimental_hashtag_search_result_limit:
     default: 20
     client: true

db/migrate/20230528134326_enable_experimental_hashtag_autocomplete_true_for_all_sites.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class EnableExperimentalHashtagAutocompleteTrueForAllSites < ActiveRecord::Migration[7.0]
+  def up
+    DB.exec(<<~SQL)
+      UPDATE site_settings
+      SET value = 't'
+      WHERE name = 'enable_experimental_hashtag_autocomplete'
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end