smp: deprecate legacy SMP core switching support

smp: deprecate legacy SMP core switching support

The deprecation was already in the documentation since v0.11.0
through commit 85ba2dc4c6ab ("rtos/hwthread: add hardware-thread
pseudo rtos") but OpenOCD was not informing the user printing a
runtime message.

Remove the deprecated method from the documentation and print a
deprecated message at runtime.
There is no reliable way to print the same message in GDB console,
so we have to rely on user noticing it in the OpenOCD log.
Target is to remove the functionality after v0.12.0.

Change-Id: Idd2d9e3b6eccc92dcf0432c3c7de2f8a0fcabe9f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6862
Tested-by: jenkins