The current ECDSAPrivateKey.SharedKey method takes a *ecdsa.PublicKey.
However, using crypto/ecdsa and crypto/elliptic for ECDH has been
deprecated in the standard library in favor of crypto/ecdh.

This commit adds a new ECDH method to ECDSAPrivateKey which takes
a *ecdh.PublicKey.  This method has the same signature
as ecdh.PrivateKey.ECDH, meaning the following interface can be be used
to do ECDH with both standard library private keys and piv-go keys, providing
the same flexibility as crypto.Signer and crypto.Decrypter:

interface {
	ECDH(*ecdh.PublicKey) ([]byte, error)
}

ECDSAPrivateKey.SharedKey has been re-implemented as a small wrapper around
ECDSAPrivateKey.ECDH.

This commit supports the new algorithms supported on YubiKeys with a
5.7.x firmware. It adds support for RSA-3072, RSA-4096, Ed25519, and
X25519.

Generating or importing X25519 keys is only supported with Go 1.20+,
which adds support for the crypto/ecdh package.

This commit removes the code to support Go 1.16 to 1.19, requiring now
Go 1.20. With this requirement we can remove the build tags.

It also renames the X25519 SharedKey to ECDH.