FEATURE: Make experimental hashtag autocomplete default for new sites (#19681)

This feature is stable enough now to make it the default going forward
for new sites. Existing sites that have not yet set enable_experimental_hashtag_autocomplete
to `true` will have it set to `false` for their site settings, which was the old default.

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

db/migrate/20230103004613_make_experimental_hashtag_feature_default_for_new_sites.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class MakeExperimentalHashtagFeatureDefaultForNewSites < ActiveRecord::Migration[7.0]
+  def up
+    execute(<<~SQL)
+      INSERT INTO site_settings (name, data_type, value, created_at, updated_at)
+      VALUES ('enable_experimental_hashtag_autocomplete', 5, 'f', now(), now())
+      ON CONFLICT DO NOTHING
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end