journalctl,shared/log: use yellow for warnings

In program output, highlighting warnings with ANSI_HIGHLIGHT is not enough,
because it doesn't stand out enough. Yellow is more appropriate.

I was worried that yellow wouldn't be visible on white background, but (at
least gnome-terminal) uses a fairly dark yellow that is fully legible on white
and light-colored backgrounds. We also used yellow in many places,
e.g. systemctl, so this should be fine.

Note: yellow is unreadable on urxvt with white background (urxvt +rv). But
grey, which we already used, is also unreadable, so urxvt users would have
to disable colors anyway, so this change does not make the problem
intrinsically worse. See
https://github.com/systemd/systemd/issues/12482#issuecomment-490374210.