[SECP256K1] Stop treating ECDH as experimental

Summary:
Backport of secp256k1 PR809.

Depends on D8044.

Test Plan:

./autogen.sh
mkdir build && cd build
../configure --enable-module-ecdh=yes
make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8049