cmake: modules: deprecate civetweb

This module has gone unmaintained and we do not have the resources to
respond to bug reports for it.

The TSC has decided to deprecate it for Zephyr v3.1 and remove it
entirely in Zephyr v3.2 unless a maintainer volunteers to keep this
module up to date and fix bugs.

This commit generates a warning at CMake time if this module is
compiled in. The warning will appear in the cmake output in the same
place that deprecated board or SoC warnings would.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>