doc: announce removal of legacy ethdev filtering API

doc: announce removal of legacy ethdev filtering API

Deprecation of rte_eth_dev_filter_ctrl() was announced in 2016,
and confirmed last year by avoiding include of rte_eth_ctrl.h.
After 4 years, it is time to complete the removal of the API
which is replaced with rte_flow.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: David Marchand <david.marchand@redhat.com>