Add deprecation notices to the POSIX shell Toolbox

The POSIX shell Toolbox has been replaced by the Go implementation
quite a long time ago. People on several ocassions created PRs that
still update it, or end up using it by mistake when building from
source.

It was not clear that the POSIX shell implementation has been
deprecated and is no longer maintained.

https://github.com/containers/toolbox/pull/698

toolbox

@@ -15,6 +15,8 @@
 # limitations under the License.
 #
 
+# THIS IMPLEMENTATION OF TOOLBOX IS NO LONGER MAINTAINED.
+# Please, use the Go implementation instead.
 
 exec 3>/dev/null
 
@@ -2187,6 +2189,10 @@ update_container_and_image_names()
 
 arguments=$(save_positional_parameters "$@")
 
+echo "THIS IMPLEMENTATION OF TOOLBOX IS NO LONGER MAINTAINED."
+echo "Please, use the Go implementation instead."
+echo
+
 host_id=$(get_host_id)
 if [ "$host_id" = "fedora" ] 2>&3; then
     release_default=$(get_host_version_id)