Fix SameSite=None incompatible check not working

!40667 (merged) didn't actually work because we need to read the Rack HTTP_USER_AGENT entry from the request headers instead of the response headers.

Relates to #241785 (closed)

lib/gitlab/middleware/same_site_cookies.rb

@@ -30,7 +30,7 @@ def call(env)
         set_cookie = headers['Set-Cookie']&.strip
 
         return result if set_cookie.blank? || !ssl?
-        return result if same_site_none_incompatible?(headers['User-Agent'])
+        return result if same_site_none_incompatible?(env['HTTP_USER_AGENT'])
 
         cookies = set_cookie.split(COOKIE_SEPARATOR)