FIX: DiscourseIpInfo.mmdb_download incorrectly joining URLs (#26545)

This commit changes `DiscourseIpInfo.mmdb_download` to use `File.join`
instead of `URI.join` when `GlobalSetting.maxmind_mirror_url` has been
configured. This is necessary because `URI.join` does not work the way I
expect it to work when I implemented it previously.

`URI.join("http://www.example.com/mirror", "test.tar.gz") results in
`http://www.example.com/test.tar.gz` instead of our expected
`http://www.exmaple.com/mirror/test.tar.gz`. For our simple use case
here, `File.join` is sufficient.

lib/discourse_ip_info.rb

@@ -27,7 +27,7 @@ class DiscourseIpInfo
   def self.mmdb_download(name)
     url =
       if GlobalSetting.maxmind_mirror_url.present?
-        URI.join(GlobalSetting.maxmind_mirror_url, "#{name}.tar.gz").to_s
+        File.join(GlobalSetting.maxmind_mirror_url, "#{name}.tar.gz").to_s
       else
         if GlobalSetting.maxmind_license_key.blank?
           STDERR.puts "MaxMind IP database updates require a license"