utils: improve highlighting in multi-column output

The yellow background made the text unreadable for light-on-dark color
schemes. Fix by making the highlighted text just bold.

Fixes Homebrew/homebrew#45028.

Closes Homebrew/homebrew#45030.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/utils.rb

@@ -43,7 +43,7 @@ class Tty
     end
 
     def highlight
-      bold 43
+      bold 39
     end
 
     def width