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

config/site_settings.yml

@@ -2035,7 +2035,7 @@ developer:
     default: true
     client: true
   enable_experimental_hashtag_autocomplete:
-    default: false
+    default: true
     client: true
   experimental_hashtag_search_result_limit:
     default: 20