terminal-util: define grey as 256color ANSI sequence

Apparently all relevant terminals implement these sequences, including
the Linux kernel, rxvt, xterm, and of course gnome terminal. Hence it
should be OK to use them, and fixate the grey color in a way that maps
to the same color in all terminals.

Ideally we'd stick to the more symbolic colors that allow terminal
emulators to implement color styles, but this apparently doesn#t work,
since rxvt maps grey to something unreadable by default.

Note that this change has negative effects besides the non-themability
of the palette: the midrange grey this uses maps to regular white on the
linux console. However, that's probably not too bad: allowing things to
be unreadable on some terminals is probably worse than showing no color
on some terminals.

Fixes: #12482