[ruby/rdoc] Avoid potentially loading the same extension from differe…

…nt versions of the same gem

Avoids warnings like

```
/path/to/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/rbs-3.4.0/lib/rdoc/discover.rb:10: warning: method redefined; discarding old scan
/path/to/ruby/3.3.4/lib/ruby/gems/3.3.0/gems/rbs-3.5.1/lib/rdoc/discover.rb:10: warning: previous definition of scan was here
```

ruby/rdoc@e47920d8f3

lib/rdoc/rdoc.rb

@@ -544,7 +544,7 @@ def remove_siginfo_handler
 begin
   require 'rubygems'
 
-  rdoc_extensions = Gem.find_files 'rdoc/discover'
+  rdoc_extensions = Gem.find_latest_files 'rdoc/discover'
 
   rdoc_extensions.each do |extension|
     begin