Make warnings yellow (previously red)

Closes Homebrew/homebrew#41003.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Library/Homebrew/utils.rb

@@ -55,7 +55,7 @@ def oh1 title
 end
 
 def opoo warning
-  $stderr.puts "#{Tty.red}Warning#{Tty.reset}: #{warning}"
+  $stderr.puts "#{Tty.yellow}Warning#{Tty.reset}: #{warning}"
 end
 
 def onoe error