FEATURE: Add rake task to mark old hashtag format for rebake (#19876)

Since the new hashtag format has been added, we want site
admins to be able to rebake old posts with the old hashtag
format. This can now be done with `rake hashtags:mark_old_format_for_rebake`
which goes and marks posts with the old cooked version of hashtags
in this format for rebake:

```
<a class=\"hashtag\" href=\"/c/ux/14\">#<span>ux</span></a>
```

c.f. https://meta.discourse.org/t/what-rebake-is-required-for-the-new-autocomplete-styling/249642/12

lib/tasks/hashtags.rake

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+desc "Mark posts with the old hashtag cooked format (pre enable_experimental_hashtag_autocomplete) for rebake"
+task "hashtags:mark_old_format_for_rebake" => :environment do
+  # See Post#rebake_old, which is called via the PeriodicalUpdates job
+  # on a schedule.
+  puts "Finding posts matching old format, this could take some time..."
+  posts_to_rebake = Post.where("cooked like '%class=\"hashtag\"%'")
+  STDOUT.puts(
+    "[!] You are about to mark #{posts_to_rebake.count} posts containing hashtags in the old format to rebake. [CTRL+c] to cancel, [ENTER] to continue",
+  )
+  STDIN.gets.chomp if !Rails.env.test?
+  posts_to_rebake.update_all(baked_version: 0)
+  puts "Done, rebakes will happen when periodal updates job runs."
+end