[newauth] Print warning for .gitcookies file

This will print a warning and disable new auth if .gitcookies file
exists.

This ensures we don't break bots once we default new auth on while
pushing human users away from .gitcookies.

Bug: b/404613530
Change-Id: Ia2d05bb7e5d2bf11e5605a26ef492954acbd9833
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6367723
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Allen Li <ayatane@chromium.org>

git_cl.py

@@ -3902,7 +3902,7 @@ def CMDcreds_check(parser, args):
     """Checks credentials and suggests changes."""
     _, _ = parser.parse_args(args)
 
-    if newauth.Enabled():
+    if newauth.EnabledInConfig():
         cl = Changelist()
         try:
             remote_url = cl.GetRemoteUrl()