drivers: ieee802154: deprecate OT "sleep" nomenclature

"Sleeping" has a well defined meaning in Zephyr related to threading and
power management. This differs from OpenThread's definition:

- Deprecates the "SLEEP_TO_TX" capability as it is redundant and
  conflicts with all of Zephyr's nomenclature, #61227, RFC 2863, Thread
  standard and IEEE 802.15.4. This binds the API to an implementation
  detail of OpenThread, instead. See #63670 for the agreed migration path.

- Renames the "SLEEP" event to "RX_OFF" which conforms to the
  nomenclature in Zephyr, this API and IEEE 802.15.4.

Fixes: #62995

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>