Fix usage of deprecated Rack::File in Rack 3.0 (#896)

Just a quick change to replace usage of Rack::File with Rack::Files, since this has been deprecated in Rack 3.0 and will be removed in Rack 3.1 (see https://github.com/rack/rack/blob/3-0-stable/lib/rack/file.rb).

Kept backward compatibility with older Rack versions by checking for the existence of Rack::Files before instantiating it, falling back to Rack::File.

lib/apipie/static_dispatcher.rb

@@ -4,7 +4,13 @@ class FileHandler
     def initialize(root)
       @root          = root.chomp('/')
       @compiled_root = /^#{Regexp.escape(root)}/
-      @file_server   = ::Rack::File.new(@root)
+      @file_server   = if defined?(::Rack::Files)
+                         ::Rack::Files.new(@root)
+                       else
+                         # Deprecated in Rack 3.0, kept
+                         # for backward compatibility
+                         ::Rack::File.new(@root)
+                       end
     end
 
     def match?(path)