shell: telnet: Replace timer with a delayed work

A `k_timer` callback is called from the ISR context on certain devices
(nRF), which resulted in an assert in the kernel, as `telnet_send`, and
thus `net_context_send` used a mutex.

Fix the issue by replacing a timer used by the `shell_telnet` module
with a delayed work, which will execute it's callback in a system
workqueue context.

Fixes #22697

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>