wasi-nn: make `wasi_ephemeral_nn` default

[yamt]

we use wasi_nn module for wasi-nn imports by default.
i'd suggest to flip the default to use wasi_ephemeral_nn.
(and probably deprecate wasi_nn and remove it in future.)

cf. #1521 (comment)

[lum1n0us]

Agree.

FYI, @HongxiaWangSSSS is targeting the same goal. @HongxiaWangSSSS , would you mind sharing the status of the second PR?

[yamt]

(and probably deprecate wasi_nn and remove it in future.)

wasi_ephemeral_nn support was introduced by:

commit db025e457afd3846815e34b385ecd79bc3262f8d
Author: liang.he <liang.he@intel.com>
Date:   Mon Jun 17 14:58:09 2024 +0800

and has always been recommended in our documentation.

wrt releases, it has been included since WAMR-2.1.1 (2024-07)

i wonder if anyone is still using "wasi_nn".