FIX: Use Guardian.basic_user instead of new (anon) (#24705)

c.f. de983796e1b66aa2ab039a4fb6e32cec8a65a098

There will soon be additional login_required checks
for Guardian, and the intent of many checks by automated
systems is better fulfilled by using BasicUser, which
simulates a logged in TL0 forum user, rather than an
anon user.

In some cases the use of anon still makes sense (e.g.
anonymous_cache), and in that case the more explicit
`Guardian.anon_user` is used

lib/middleware/anonymous_cache.rb

@@ -169,7 +169,7 @@ module Middleware
       def theme_ids
         ids, _ = @request.cookies["theme_ids"]&.split("|")
         id = ids&.split(",")&.map(&:to_i)&.first
-        if id && Guardian.new.allow_themes?([id])
+        if id && Guardian.anon_user.allow_themes?([id])
           Theme.transform_ids(id)
         else
           []