ckem: move crypto/elliptic to crypto/ecdh

[MingLLuo]

TODO move over to crypto/ecdh once we can assume Go 1.20.

I found that there is an unfinished TODO in ckem.go, and the go version we use now is higher than 1.20.

Due to the limited code using crypto/ecdh in the entire repository, I can only go by my own experience with the transition, so some of the implementations may not be so satisfying.

This also help fix some problem of

TODO: replace deprecated elliptic.Marshal, elliptic.GenerateKey,
elliptic.Unmarshal, params.ScalarBaseMult before re-enabling SA1019.
in circl/.golangci.yaml

[armfazh]

Thanks @MingLLuo