What do you think about renaming this from `encryptionKey` to `encryption_key` using the legacy deprecation system? Reading the full config path, `xpack.encrypted_saved_objects.encryptionKey` looks rather inconsistent with the mixing of `snake_case` and `camelCase`. I know you didn't technically change this as part of this PR, but maybe the NP migration is a good time to start to make these types of things consistent?