nyan: deprecate plugin with removal notice

Fixes #6826

plugins/nyan/nyan.plugin.zsh

-if [[ -x `which nc` ]]; then
-  alias nyan='nc -v nyancat.dakko.us 23' # nyan cat
-fi
-
-
+print -Pn '%F{yellow}'
+cat >&2 <<-EOD
+	nyan plugin:
+	The nyancat server used by this plugin was shut down due to increased
+	bandwidth costs, so the nyan plugin no longer works. You can get the
+	same functionality in some distributions by installing the nyancat package,
+	or you can compile it yourself.
+	See https://github.com/klange/nyancat for more information.
+EOD
+print -Pn '%f'