doc: Suggest keys.openpgp.org as keyserver in SECURITY.md
doc: Suggest keys.openpgp.org as keyserver in SECURITY.md

This is in line with https://github.com/bitcoin/bitcoin/pull/23466/ .

bitcoin/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1,bitcoin-core/secp256k1,ElementsProject/secp256k1-zkp,bitcoin-core/secp256k1,bitcoin/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1