net: socket: service: mark as unstable

As the socket service API is currently used by mutiple applications
(dhcpv4 server, dns, telnet), it should be marked as unstable,
according to the docs:
https://docs.zephyrproject.org/latest/develop/api/api_lifecycle.html



Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>